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The question of what class the PC should be comes down to where are the NPCs deficient. But by and large there are excellent NPCs for every role:

Tank: Kagain can take damage better than anyone. He has great saves. And he regenerates. Give him the gauntlets of dexterity and you can't build a better tank.
Archer: Either Kivan or Coran is excellent. For fighter who deal out lots of damage both of these could be used. You can make a PC who is a bit better (19 dexterity, 4 stars in bow etc.) but really they are very good.
Cleric: Viconia is a spectacular choice - the magic reistance is worth a lot. As a fighter/cleric you can build a PC who is better than Yeslick, but really you don't need more than Viconia.
Mage: Again there is a spectacular choice in Edwin. His hit points suck, but the spell selection and number of spells can't be beat by a PC. The other mages are all substandard - Dynaheir because invokers miss out on all the best spells, Xan has Edwin's drawback and none of his advantages, Xzar suffers immensely from not having mirror image. Quayle is OK, but you could build a much better C/M PC. Imoen dualled is, of course, almost as good as Edwin, and basically as good as any PC you can build.
Thieves: Here the NPCs aren't as obvious. Coran is great as an archer, both Coran and Montaron are good (bit not great) back stabbers, Imoen and Safana are very good except for backstabbing.

To me the holes in the NPC group that the PC can readily fill are:
-Mage who can fight, use mirror image/blur to cause all sorts of havoc. Calls for a F/M or F->M. None of the NPCs is close to doing this. Eldoth and Garrick simply can't match a F/M.
-Spectacular backstabber. F/T or F->T. Coran and Montaron both fall a little short here.
-Cleric who can best use cleric buffs. F/C or F->C.

Because of the way the game progresses I think the dual classes are quite a bit stronger - you don't need much spell power until later, and later on you need your casters to be able to take same damage.

I think my powergaming party would be:
-Kagain, tank and decoy.
-Coran archer, scout, lock picker
-Imoen traps, pick pockets, and back line mage (dual at lvl 5 or 6, once traps are at 100% and pickpockets are close enough to get the rest with potions)
-Viconia cleric extraordinaire
-PC fighter dualled to mage at level 7

The trick here will be managing both dual classes. However it's easy to add another thief until you get Imoen's traps back.

If you're taking a character into BGII it's unlikely you'd want to dual at BG levels. But for BG alone these are the most powerful characters at the end of the game.