I'd have to say that the most powerful class all the way through would be a fighter/thief multiclass. In BG1, backstab really shines, and melee is very important - the fighter/thief is very good at both. His thief levels level up quite fast and the fighter xp table is no slouch either. In BG2, it's undoubtedly slightly less powerful than other classes out there (solo) but then come in the HLAs...great selection there. All those combat boosts from the fighter side, mixed with the also very good thief section: think assassination, Use Any Item (a truly awesome ability).
I say this over fighter/mage/thief because in BG1, the extra class definitely hampers you a lot and gives you little in return, and your reward doesn't really come till around the underdark in BG2, however from then onwards it's a very formidable class. However, the fighter/thief has a constant and relatively unchanging high amount of power through each game.
Fighter/druid(multi) definitely is up there with fighter/thief with the nice spell selection, higher hitpoints and self-heals but with rings of regeneration and the large amount of potions available freely, it's slightly less. Having said that, I prefer playing fighter/druids or [fighter/ranger]/clerics over fighter/thieves.
The Blade is a worthy candidate and levels up very quickly indeed, which is a real plus, but missing trap skills, hp, constant fighting abilities, backstab and the choice of two HLA pools, I wouldn't rate it as highly. You can't doubt its buffs though...tensors transformation + offensive spin + stoneskin + mirror image + whatever the hell else you want =
My view.
