Fighter/Thief all the way! You can be a Half-Orc and start with 19 Str and 19 Con. That's some nice bonus damage to throw into a backstab multiplier
As a Fighter/Thief you'll get three of the best High Level Abilities in the game: Greater Whirlwind, Use Any Item and Spike Trap.
With some Pickpocketing skill and Use Any Item you've got a lot of wands at your disposal.
Finally a Fighter/Thief is a LOT of fun as a scout. You can sneak up ahead and take care of small problems by yourself.
-- Nifft