To be honest with you I always found a pure wizard (general, not a specialist) to be the easiest, and most fun for me, character to play the game with. Especially when you get companions to help you in the single player game.
With a high enough intelligence you can get the skill points to help you get the skills to pick locks and disarm traps, you get summons that can be your meat shield and you get a familiar to help you fight.
Get a few levels under your belt and you start to get spells that are just devastating if used correctly.
Polymorphing into a dragon and summoning a Balrog, or a celestial, to fight along side you while kicking but is very, very cool.
On player worlds a Cleric, Monk, Shadow dancer is pure cheese and, if built correctly, can not only handle the toughest challenges most builders come up with but can also pawn in PvP.
Even so I'd take the wizard. With the right combination of skills, feats and spell selection a wizard can handle any situation. PvE or PvP.
I've played with just about every combination of classes and builds you can think of and had a ton of fun with most of them.
The main thing is to come up with a character that you think is fun to play because even if you screw the character's build up NWN has enough magical items and weapons to let you adjust where you have to and be able to make it through the SP game.
And on a PW you can always hook up with a group and go adventuring together so you'll, most of the time, have someone covering your back anyway.