Old world islands.
Pirate starter island (for lv 0-10)
Bandits spawn here, and a sword dealer and the blox fruits stock.
Marine Starter Island (for lv 0-10)
Trainees spawn here, and there is again, a sword dealer and a blox fruits stock.
Middle town has a blox fruit dealer, a weapon dealer, and has an enhancement editor, and has the npc quest for Yoru v2. Saw also spawns here.
Jungle (for lv 10-30)
Monkeys and Gorillas spawn here, as well as the first boss in the game, Gorilla King. There are a few plates here and some chests, and there is also shanks.
Buggy island (for lv 30-60)
The enemies here are pirates, brutes and Bobby, the second boss in the game. There is also another sword dealer. There are 3 gold chests and one blue chest, making this a good money farm early game. You can also buy dark step from here. It plays a good role in the Shank's quest.
Desert (for lv 60-90)
The two npcs here are the desert bandits and the desert officers. The desert also plays a good part in the shanks quest. You can also buy the swordsman hat here.
Snow island (for lv 90-120)
There are 4 npcs here. There is snow bandits, snowmen, yeti, and ice admiral. The island is an important role to the shanks quest, and is also where you find love letter one for the Yoru v2 quest. There is also another sword dealer here.
Marineford (for lv 120-150)
Marineford has two npcs here. There is chief petty officers, and there is a boss, vice admiral. This island also has a raid boss, and also has a gun seller here. You can find love letter number 3 here for Yoru v2 quest.
Skypiea (for lv 150-200)
Skypiea is made up of 4 islands, the first four sky islands. There are two npcs being the sky bandits and the dark masters. You can find the master sword dealer, and you can also find love letter number 2 for the Yoru v2 quest. You can also buy electro fighting style.
Prison (for lv 200-250)
Also known as impel down, there are only bosses, and there is no spawn point here. The three bosses are Warden, Chief Warden, and Swan (not Don Swan, that's in new world.) There are chests here, and there is a npc that will get rid of your devil fruit for 50000.
Colosseum (for lv 225-300)
This is the place where most old world PvPs will happen. There are two npcs, being the toga warriors and gladiators. There are a few chests.
Magma Village (for lv 300-375)
Magma village has three npcs. Their is military soldiers, military spies, and the boss, magma admiral. You can also buy soul cane, which is good for making combos.
Fishman island (for lv 375-450)
Fishman island is very hard to identify, mainly because the entrance is in the water, and all thats surrounding it is 3 rocks, but this is the closest spawn point to prison as well. Fishman island has three npcs, fishman warriors, fishman commandos, and fisherman lord. You can also buy fishman karate here.
Upper yard entrance (for lv 450-475)
Upper yard entrance is the fifth island of the six and has one npc which is the god's guard. You can also break the cloud in the temple to reach the sixth and final island in sky islands.
Upper yard first section has two npcs, being the shandas and the boss wysper. The second section has three npcs, royal squad, royal soldier, and the boss thunder god.
Fountain city (for lv 625-700)
This is the last place before new world. There are three npcs here being the galley pirates, galley captains, and the boss cyborg.
After you reach level 700, you get a message saying congratulations on hitting level 700, and asks you if you're ready to go to the next world. But this is the island page, so we are skipping this stuff.
Kingdom of rose Area 1 (for lv 700-775)
This is the first three quests in new world, being raiders, mercenaries, and the first boss in new world, Diamond. There is also a blox fruit dealer when you first get to new world
Kingdom of rose Area 2 (for lv 775-875)
The second area in new world, where the cafe is. Has 4 quests. There is the swan pirates, factory staff, and Jeremy. There is also the factory raid.
Green zone (for lv 875-950)
There are three npcs here. There is marine lieutenants, marine captains, and fajita. There is the alchemist, who will sell you your race v2.
Graveyard island (for lv 950-1000)
There are two npcs. The zombie and the vampire.
Snow village (for lv 1000-1100)
Two npcs which are the snow troopers and winter warriors.
Hot and cold (for lv 1100-1250)
There are two islands, and there are 5 npcs total on both islands. The npcs are lab subordinates, horned warriors, magma ninja, lava pirate, and the boss smoke admiral.
Cursed Ship (for lv 1250-MAX)
Finally, the newest island as of update 12, we have the cursed ship, which has 5 npcs, one is a raid boss. The four regular are the ship deckhand, ship engineer, ship steward, and ship officers. The raid boss is the cursed captain.
Here you summon blackbeard.
Island that sells kabucha.
That weird rock that apparently has no use.
NOTE: Recommended to grind bosses starting from impel down all the way to max level.