Err. Considering the small amount of "Hack n' Slash" MMOs out there(that I know of), it'd probably be better to list them in a particular order.
3D List
1. Dragon Nest (CBT is starting soon, however I played the Korean version and it was phenomenal.)
1.5. Vindictus/Mabinogi Heroes (Right up the alley with Dragon Nest, however it's more mature themed compared to DN)
2. Divine Souls (I didn't particularly like the instances in this game. They felt too drawn out and less polished)
3. GunZ (I wasn't a fan of the game since it's almost entirely PvP, but it has a large playerbase)
I don't think these fit into the "Hack n' Slash" category, but oh well
2D List
1. Dragon Saga (Polished instances, though they can get repetitive at higher levels, Great skill animations. Lots of fun.)
2. Grand Chase (Though the instances are a tad repetitive, with the new Chaos update it's a lot easier to level and acquire new characters)
3. Lunia (Great instance system. However, it requires Cash Shop items for more than one occasion. Ex. Characters, rebirthing, etc.)
4. MapleStory (Large playerbase. Leveling became easier after the Big Bang update. No instances, however there are party quests.)






