The only class I would consider playing 1-20 without multiclassing is a Cleric.
Rogues, Rangers, Fighters, and Barbarians are going to be popular multiclass options because their power doesn't increase exponentially like a spellcasting class does. Taking a level of fighter, for example, really helps your character (especially a low level spellcaster) without hindering their progression too much (19 wizard instead of 20). However, a 19 fighter/1 wizard really won't do much for you.
I think because your party in NWN is all of two people, and one of them is going to be a specialist, multiclassing is kidn of implied in the game design. Plus there is nothing like putting that greatsword aside to launch a fireball and then charge into combat. HUZZAH
