It turns out that AD&D based games actually do have best classes, largely due to the fact that any given game's environment tends to favor certain classes. Here are some of them (considering only single classes):
Baldur's Gate: Fighter with a focus on bows. (Slow walking speed + availability of powerful arrows + spells being limited at low levels and area spells being hard to use.)
Baldur's Gate 2: Wizard or (if you're already familiar with the game's spells) Sorcerer. There are tons of powerful arcane spells, including some that you don't get in other AD&D CRPGs, like (Chain) Contingency, Project Image (which I believe is more powerful than its AD&D counterpart, and is almost as strong as the Phantom Warrior spell from Romancing SaGa), Time Stop (intended to appear in a couple SSI games, but not always available/functional), and even Wish. Also, if you are willing to use scrolls (which are plentiful, though typically finite) as consumables, you can also get around the game's level gating of higher level spells (or you could use wild magic for this).
Icewind Dale: Cleric. Undead are common in the game, and Clerics, unlike Wizards, are not affected by the scarcity of scrolls and lack of level-up spell picks.
For some of the older SSI games:
Pools of Darkness: I believe it's magic-users here, especially since, at high levels, they get the ability to pierce magic resistance. The spell selection is rather lacking at some spell levels (there's no spell above 7th level that's as good as Delayed Blast Fireball, for example).
Dungeon Hack: Cleric. Wizards have the same issues as in Icewind Dale (plus the fact that the spell you want isn't even guaranteed to be in the dungeon), while Clerics get spells to negate many of the game's dangers, including hunger and poison.
Dark Sun: Shattered Lands: Probably Psionic. Psionic powers are not level gated the way spells are. You can, for example, get Mass Dominate after the first level up and break the game that way. (I highly suspect that a speedrunner's party would include at least one psionic for this reason alone.)