We all know that hunters are beast at low levels(2moons gold) because it can get mine to 50 in less than half a day of starting it. But summoners are fun to level also at low levels with more lure and some aoes. Once you get higher it really depends on how well you can get pts since grinding is a pain.
Don't try to rush your levels, with the new xp curve and quest rewards in 2Moons, level 1-90 goes by so fast! But after that everything sucks. Depending on your playing style, different classes are faster for different reasons. In early levels, mages have the most AoE skills so can mob faster, they also get DF parties pretty easy, so can level fairly fast.
Summys can use their pet to lure extra mobs, so can complete Commission Quests faster (if you have to solo), hence faster leveling(2moons powerleveling), but may struggle to get DF parties. Hunters on dagger or heal build can solo pretty much everything with ease, including the level60 dg in Crespo, so can level fast.
Segs are very tough until getting Curse Field (around level 60 i think), but will get parties easily due to healing skills. Bagi's are notoriously hard to level due to their low level skills, although we are not very experienced with this class. Also AKs and Alokens are new to us, so cant give much advice there.