The class tier list was my best attempt to balance overall usefulness (solo and party). Tiers S, A, and B are all pretty good soloists, though. Most of my recent playthroughs have been solos.
I’ll go ahead and make an NPC Tier list for BG2, while I’m at it:
Tier S:
Edwin (Conjurer) – Best pure Mage in the game, an even better caster than the PC could ever be. Starts with an 18 intelligence. Conjurer is the best specialist, as a specialist he gets an extra spell per level, and on top of that, his amulet grants him an additional spell slot per level, which gives him as many castings per day as a Sorceror, but with the flexibility of a Mage. Beat that.
Keldorn (Inquisitor) – His class says it all. Mage-raper extraordinare, and also a great party leader to boot thanks to his compulsory high charisma. He’ll also be able to summon Devas when his levels get high.
Tier A:
Imoen (Thief 7/Mage) – Best good-aligned NPC Mage, and the second-best NPC Mage overall. Also comes with sufficient thieving skills to handle the entire game and the expansion. You’ll get an opportunity to boost her intelligence to 18 in the game.
Korgan (Berserker) – Best pure fighter. He’s chaotic evil, though.
Jaheira (Fighter/Druid multi) – Gets both Fighter and Druid HLAs, which means she’ll be able to wade into melee with Greater Whirlwind alongside some Elementals. Iron Skins, Insect Plague and Creeping Doom spells round out the package. She’s also the only Druid that can cast Raise Dead, after an extensive sidequest. Also the best romance, by far.
Valygar (Stalker) – If you can get past his lifeless personality, the combat versatility and the backstab inherent of his kit can prove very helpful. He could’ve had a better racial enemy chosen, though.
Sarevok (Fighter, ToB only) – Close second to Korgan as best pure fighter, only because Korgan has a better kit. Sarevok comes close because of his big-time damage Deathbringer Assault that can come up, plus his SUPERB stat line.
Tier B:
Aerie (Cleric/Mage multi) – Access to Cleric and Mage spells is great. But because it’s a multi between the two, her spells in either class list aren’t as numerous or as powerful as they would be for a more focused caster, i.e. Edwin, Imoen, Viconia, Anomen, or a PC dual-class Mage or Cleric. Her low constitution is also a major concern.
Jan (Thief/Illusionist multi) – Illusion spells aid his thievery, and he’s the only character in the game that can get Thief HLAs and improve his stealth and backstab (another part of his game also helped by his spells). If you use him and think of him as a Thief with substantial magical aid, and not as a full caster or all-out battle mage, you’ll find him plenty useful.
Viconia (Cleric) – Best pure NPC Cleric. Gets magic resistance for being a drow. She’s still not as good as a dual or multi with a warrior class, though, and she won’t light the world on fire in physical combat (this ain’t 3rd Edition).
Anomen (Fighter 7/Cleric) – Starts out mediocre at everything. If you do his sidequest right, he can wind up a decent caster Cleric with decent fighting skill. Better fighter than Viconia, but still pales in comparison to any decent PC dual or multi with a warrior class. Also falls behind Jaheira in melee at higher levels, since, unlike Jaheira, Anomen lacks Fighter HLAs.
Tier C:
Minsc (Ranger) – Yeah, yeah, he’s the loveable idiot with the hamster, but let’s face it: why is he a Ranger? He’s not really geared to be used as such. The vanilla Ranger isn’t nearly as effective as an Archer or Stalker to begin with, leaving him the utility of a typical Fighter with a few spells. His special berserk ability is uncontrollable and not quite as useful as a Berserker’s or Barbarian’s. He’s not BAD, but compared to Keldorn, Valygar, Sarevok, or Korgan, he falls a bit short.
Mazzy (Fighter) – The halfling is a grandmaster in short bows and also has limited use of the Cavalier’s Resist Fear and (sort of) the Stalker’s Haste. She can have her uses, but in a well-balanced party there’s not too much of a place for her. Any Archer will be better with a bow, and Valygar’s Haste affects the whole party, not just the caster.
Nalia (Thief 4/Mage) – Imoen-lite. A pure mage with a few thieving skills, but unlike Imoen, who supplies all the thieving skills a party will need for the game and its expansion, Nalia’s skills are far from adequate for that, and you’ll never be able to improve them. Use her during the part of Shadows of Amn that you may need her for, but Imoen or (if you’re evil) especially Edwin are far better otherwise.
Tier D:
Haer-Dalis (Blade) – For some reason, I’ve never seen the appeal in Blades. Sure, they’re good at fighting for being Bards, but they’re still not fighters. He can’t really do anything that Jaheira can’t in terms of spell buffs and combat, and a battle mage he’s definitely not. He gets killed easily too, thanks to his low constitution.
Yoshimo (Bounty Hunter) – Well, on paper, he should be the best thief NPC in the game. Then something happens…
Cernd (Shapeshifter) – Single-class Druids are good at getting killed in combat. Especially if they don’t have good dexterity scores.