I'd say the best main for all purpose is 1 of these 3 :
Has Devastation Shell, good combo rate, and able to snipe if BS-ed.
what more can i say? cannon main is great at PvP, PvE, and TBS.
Strong spiking power and good tanker.
although more prefer sword main in PvP
because of champion.
saw main rocks at PvE, and also good at TBS.
Staff main's Flame blow can turn the tides of battle in PvP.
althought staff main is pretty weak to snipes, so you need to take care of your main char well.
in TBS, flame Blow rocks. with 5x5 area and 40k+ damage.
although in PvE
main staff will mostly heal & guard.
please take note that these are my opinion.
other ppl may think differently.
other good mains are:
Best Tanker in PvP, WR + CE hurts pretty much. althought not much of use in TBS & PvE.
Good in PvE
& TBS, IMO RM is the best AoE
debuff in the game.
althought in PvP, RM just tickle a bit. but melody of madness can be pretty annoying.
other main aren't too good.. YET.
in kAO, bow main can learn hwarang aura, so you can kick your hwarang.
spear main will get buffed, get more combo rate and spear will add Vit.
also can learn Guan Yu's Green Dragon's Rage & War Cry.
gun main can learn steady stance and summon machine.
althought in korea bow & staff main seems is the favourite right now,
perhaps the US & Indonesia server could give more variation of forms later, when the patch comes to us.