First, it's the exp per hour that's most important, not exp per monster. This is how it works - you may be able to kill 5x as many of monster A with 1/3 the exp of one monster B, in the same amount of time it takes you to kill that one monster B. It adds up to more exp per hour.
Second, you need to use your own judgement based on your resources and skill. These are suggestions - some may be too hard for you, and some may be too easy. If that's the case, go somewhere easier / harder respectively. Take into account your healing items, equipment, build, and if you're solo or partying. Of course, you should to look up the dungeon or monster before you go there if you haven't been there before. Not every monster on every mentioned map is listed, so here are resources:
Not every dungeon, map, or monster is included in this guide, mostly the New World maps.
In addition to these places, there is also the TamTam's Gift map each month. All monsters on these maps have boosted exp in addition to a chance to drop Copper Coins and Bronze Coins.
Part 1: Low Levels
These are places suitable for level 1-45:
Anthell - Ant Eggs to begin with, then the various ants. The ants will mob you if they see one of their colour attack you. Be careful around the Vitata until around level 30.
Prontera Culvert - Volunteer in the North West building in Prontera. Kill everything on level 1, recommended for level 12+. Level 2 is recommended for level 25+, level 3 for 35+.
Payon Dungeon - This is good for Acolytes, as soon as you get Heal use it on the undeads. Level 2 is good for 35+.
Muka - These are located 2 South of Morroc. Good at level 20+. Watch out for Dragonfly.
Spore - Various maps, but a good one is 1 East of Payon. Recommended for level 12+.
Smokie - 2 East of Payon. These are good for about 40+.
Rocker - 1 West, 1 South of Prontera. Good for level 15+.
Creamy - 2 West of Prontera. Great for 25+, and also for the card and Silk Robes. This map has Rockers as well. Repeatable Quest.
Roda Frog - Best map is 3 West of Prontera. 12+.
Drops - Several maps, best map is 1 North from Morroc. This map also has Peco Peco Eggs, both are good for cards. Level 1+.
Lunatic - Highest spawn is 2 North from Prontera ( go through the Castle and WoE map ). Good for level 1+.
Pupa - 1 North from Geffen. Good for level 1+, does not attack unless it transforms into a Creamy.
Poporing - Several maps, use RMS to find the best for you. Good for level 6+.
Peco Peco - 3 South from Prontera - go through the Eastern bottom warp on the second map. These will assist just like ants. Good for level 20+.
Hode - 2 South, 1 East of Morroc. This map has Frilldora and the MvP Phreeoni on it, so bring flywings. Good for level 25+. Repeatable Quest.
Leaf Cat - Warp to Ayothaya Dungeon, or go through the East exit from Ayothaya. Tank recommended for lower levels, good at 40+ otherwise. Repeatable Quest.
Metaling - 1 North of Lighthalzen is the best map. These can hit you pretty hard with Mammonite, so a tank is recommended until you can survive it. Level 40+.
Savage - 2 South, 2 West from Prontera. Level 25+, also Savage Babes on this map for additional easy exp.
Porcellio - 1 South, 1 East and 2 East from Einbroch. Drops a great card, good for 35+.
Thara Frog - 1 East from Comodo, in the cave. Good card, though the map may be full. Good for 30+.
Sandman - 2 South, 2 East from Morroc. These are aggressive, will hide, and will mob you easily. 40+.
Flora - 1 East, 2 North from Prontera is the best map. Good for Mages and Archers. Level 35+ ( or whenever you can hit them ).
Caramel - 2 North from Geffen, or 1 North, 1 East. Good for 25+. Repeatable Quest.
Wolves - 2 South from Payon. Good for 35+.
Part 2: Mid-low Levels
These are places suitable for level 45-65:
Coal Mine - Level 2. Kill everything, bring lots of flywings. Good for level 45+.
Orc Dungeon - Both levels. Start at level 45, then go to the next level around 55.
Sunken Ship - Various monsters here, just stay on level 1. Good for Hydra cards. Good for 45+.
Toy Factory - Both levels, but 2 has more mobs. Go here once you have enough hit for Myst Case, good for making zeny. You should be able to go here around 45.
Brasilis Field - North portal in Brasilis city. Good at 55+.
Argiope - Best map is 2 South from Aldebaran. These are best for ranged or Mage classes. Bring flywings for quick escapes if needed. Good around level 45+.
Kobold - 2 West, or 2 West 1 South from Geffen. Easily mob-able. Level 45+.
Goblin - 1 South, or 1 South 1 East from Orc Dungeon. Good at 55+.
Orc Lady - Outside Orc Dungeon. Good from level 45+, and Orc Babies from 60+.
Grand Peco - 2 North, 1 West from Aldebaran. These will mob if they see one attacking you, watch out for Sonic Blow. Level 60+.
Amatsu Dungeon - Quest required to enter. Level 1 is good for Acolytes once you get Pneuma, and level 2 is good for everyone else. Start when you get Pneuma, otherwise good at 50+.
Biblian Dungeon - Level 3 for Obeaunes, good for level 45+. Level 4 for Phen, Marc, and Swordfish, all good cards. Level 55+.
Einbroch Dungeon - Pitman, Venomous, Noxious, watch out for Ungoliant and traps. Good at 60+.
Glast Heim St. Abbey - Southern middle building in Glast Heim, or go through the South exit at the Churchyard. Great for Acolyte or Priest, with a good card drop from Evil Druids. Recommended level 60+.
Umbala Dungeon - North warp at Umbala city. Good for level 65+.
Moscovia Dungeon - Finding the Moving Island is the entrance quest. Level 1 for 45+, level 2 for 60+. Repeatable Quest.
Clock Tower - Walk straight up from the spawn in Aldebaran. First map is Punks, good for melee level 60+. South East warp is to Clocks, good for magic and ranged level 60+.
Geographers - Best map is 1 North from Einbroch. Good as soon as you can hit them, about level 45+.
Muscipular and Drosera - Found all over Veins fields, best map is outside Ice Dungeon. Watch out for Gryphons. Standing in a straight direction from the Drosera is the best tactic. Good as soon as you can hit them, about 45+.
Alligators - 1 East from Comodo through the Thara map and through the South East exit, or even better Dungeons -> Kokomo Beach then north. Good at level 55+. Repeatable Quest.
Ice Dungeon - Level 1 recommended, level 45+.
Mi Gao - 1 South from Louyang. Great for magic and ranged, level 50+.
High Orcs - Middle West warp from Orc Dungeon entrance. Recommended level 55+.
Part 3: Mid to Mid/High Levels
These are places suitable for level 65-85:
Goats - 2 South from Yuno. Watch out for Harpies. Good from level 70+. Repeatable Quest.
Kokomo Beach - 2 East ( through Tharas ), 1 South from Comodo. Mobsters are good for zeny. Good at level 70+ though you may have difficulties with Mobsters until 80+.
Pyramids - Warper or 1 North West from Morroc. Level 4, good for mobbing. Watch out for Osiris. Recommended level 80+.
Sphinx - Warper or 1 West from Morroc. Minorous and Pasana are best for melee and magic, Anubis best for TU / ME or magic. Level 3 is good at level 65+. Level 4 good at 70+. Level 5 recommended level 80+, watch out for Pharaoh.
Alarms - Clock Tower, middle-top warp on the first level, then top center warp. Good at level 70+.
Turtle Dungeon Outside - You can easily get mobbed here. Recommended level 75+.
- Turtle Dungeon - North warp at Turtle Island. Recommended level 80+.
Groves - 1 South, 1 West, 1 North from Hugel. Okay at level 75+ due to the Novus on the map.
Dryads - 1 North from Comodo through Stalactic map, East exit. Good at level 65+. Repeatable Quest.
Parasites - 1 East from Comodo through Tharas, South exit, then North West exit at Alligators. Watch out for Gryphon. Recommended 70+.
Hill Winds - Several maps, best is 1 South, 1 North East from Ice Dungeon entrance. Good for zeny. Recommended level 70+.
Petites - Several maps. Sky Petites are at 2 North, 2 East from Geffen. Earth Petites are 2 North, 1 East from Geffen. Recommended level 70+ and 65+ respectively.
Demon Pungus - 1 South, 1 West, 1 North from Yuno. Great for mobbing. Good at level 75+.
Biblian - Level 5. Water armor necessary for Waterball. Great for level 75+.
Wind Ghosts - Southeastern building at Glast Heim ( looks like an entrance to an underground area ). Teleport necessary. Great at level 75+.
Stings - Warp to Glast Heim Churchyard, go through the West exit, then teleport to the top right exit. Watch out for Gargoyles if you can't flee or kill them. Very nice card drops. Great at level 75+.
Majoruros - Glast Heim, 3 maps past Stings. The walkways are tight, the best method is mobbing and killing with AoE magic. Good at level 85+.
Magma Dungeon - Level 1 is best for magic users, level 2 is best for melee but magic can be used as well. Level 1 can be done solo with a Wizard. Recommended level 80+ for either map. Repeatable Quest on level 2.
Louyang Dungeon - North West exit from Louyang, then around the "back" of the ruins in the West. Level 2 and 3 are both suitable for 75+.
Part 4: High Levels
These are places suitable for level 85 til 99:
Ice Dungeon - Level 2 or 3. Level 3 recommended over level 2. 85+.
Biolabs - Requires an entrance quest. Level 2 is good for 85-90. Level 3 is restricted to level 95+ non-trans and 90+ Transcendant. Party is very much recommended for level 3.
Juperos - Level 1 or 2 recommended for 85+. Going to the Core requires a quest each time, and is recommended for 90+.
Moscovia Dungeon - Requires Finding the Moving Island quest to enter. Note: Kill everything for the best zeny-making. Level 3 is good for zeny but light on exp at 85+.
Rachel Sanctuary - Requires an entrance quest. All levels are good from 85+.
Penomena - Clock Tower, center-top warp, then South West warp. Good at 85+.
Abyss Dungeon - Requires items to open the portal. Party recommended, but can be solo'ed. Level 90+ is best.
Nifflheim - 1 West from Nifflheim city is recommended for 85+ with a party. 2 West from the city is recommended for soloing at 85+.
Galions - Several maps, best is 1 South, 1 West from Ice Dungeon entrance. Watch out for Atroce, but you should be able to kill him with a decent party. Best tactic is mobbing, watch out for pesky Drosera that interrupt casting. 85+.
Anolians - 1 East from Alligators or 1 level past Stings. Mobbing with an Assassin is best. 85+.
Sea-Otters - 1 East from Seals. Can be solo'ed or done with a party. Okay at 80+.
Ninetails - 2 South, 1 West from Payon. Recommended mobbing and killing with a Wizard. Also kill the Dragontails, and watch out for Eddga though you should be able to kill him. Great for 85+.
Tamruan - Ayothaya Dungeon level 2. Requires an entrance quest. Bad job exp. Watch out for Lady Tanee. Good for soloing at 85+.
Stalactic Golem - North exit in Comodo. Best for soloing with Claymore traps. *Note: Claymore doesn't scale based on mob size*. Good at 85+.
Medusa - West exit in Comodo. Good for mobbing with Assassin, useful card drop. 85+.
Cornus - 1 East, 1 North from Splendide. Cornus are non-aggressive until you're level 99. Naga will likely 1-shot you, so watch out. Best for AD Biochemist or Beast Strafe Sniper since they can 1-shot, otherwise it will fully heal itself. Best at 85+. Note: Pinguiculas are easy and can be done at 75+.
Additional Party-Recommended, High-level Dungeons
These maps require a degree of skill and proper equipment. Research before exploring:
Thor's Dungeon - 1 North, 1 West, 1 North from Veins ( make sure you're on Veins Field 3! ). There's a quest that allows you to bypass level 1, not required. Party recommended for level 1 and 3. Watch out for Ifrit on level 3. Great at 90+.
Cursed Abbey - 1 South of Veins, talk to the NPC at the dock in the South. Requires 5 total quests to gain access, all linked in the guide. Level 1 is good for soloing, level 2 is fine for mobbing, Level 3 is highly recommended for leveling with a party, but is solo-able by ME priests with instant cast. Great for 90+.
Dimensional Gorge - 1 East from Morroc, then talk to the NPC immediately inside. Requires an entrance quest, and you must be in a party of at least 2 people to enter the map. Party highly recommended. Great for both base and job exp, level 90+. Asura, Sharpshooting, or a Star Gladiator are great here. However, this map requires high-tier gears.
Geffenia - Do the Sign Quest to be able to access it anytime, or ask someone to open the portal, located directly South of the spawn in Geffen city. Solo-able and great for zeny-farming. Level 85+.
Odin's Temple - Fields 2 and 3 for parties. Talk to the man on the dock in Eastern Hugel. Party is very highly recommended, with both magic and melee. Best for level 90+.