Some of the most powerful BG2 race/class combos are...
Human Inquisitor: The inquisitor is the best paladin class kit. Plus, if you take this class you'll be able to use the Holy Avenger, which is one of the single most powerful weapons in BG2.
Half-orc Barbarian: The barbarian really is a powerful class in BG2; he's immune to backstabs, he's fast on his feet, and his rage ability makes him immune to things like domination and level drain. Pick half-orc as your race so that you can get 19 Str and 19 Con. Put as many proficency points as you can in two-weapon fighting, and then you can fight with the Celestial Fury katana in one hand and the Equalizer longsword in the other. Alternatively, you can replace this with the Berserker class kit, which is similar to the barbarian.
Human Monk: As Henry said in his post, the Monk class in BG2 is absolutely devastating. Although they aren't real powerful when the game begins, at higher levels they are easily one of the most powerful classes in BG2. You can't go wrong playing as a monk.
Elf Archer: The Archer is the best ranger kit in the game, and capable of dealing lots of damage from range. Max out your proficency in shortbows so you can use Gesen's short bow (which never runs out of arrows) or choose longbows and pick up the Quiver of Plenty in Throne of Bhaal. Since elves get bonuses when using bows and have high dexterity, they are usually the best choice for the archer kit.
Half-Orc Kensai: The Kensai is the best fighter kit, and is capable of dealing whopping amounts of damage. As with the barbarian, choose half-orc as your race so you can get 19 Str and 19 Con. Max out your proficencies in either katanas or longswords. You can also max out dual-weilding and fight with both types of weapon.
Human Kensai/Mage: Since neither kensai nor mage can wear armor, these two classes work excellent together (fighter/mages can wear armor, but can't cast spells while the armor is worn). You'll get the attack and damage bonuses of the kensai, in addition to being able to cast spells. The only class that can take advantage of this combo is a human (with 15 Str and 17 Int) who begins as a Kensai, and then dual classes to a mage. Take proficency points in Katana so you can take full advantage of Dak`kon's Zerth Blade, which is the perfect weapon for a kensai/mage.