Tiax rules over all! ...not.
Tiax, a low wisdom Cleric / Thief has one of the worse class combination ever. In BG:EE, I wish that extended classes options be implemented and that those NPCs currently bearing boring classes be changed.
A Druid / Thief ! That's cool! Also, where's the Fighter / Bard ? It's as if the AD&D's PBH has forgotten about all of these great class combinations only because those classes are optional. Thief / Illusionist? Hey, that's like the single mage specialization that can be multiclassed. It's not fair! Well, why not make some kits possible in multiclassing options? Actually, I think it would be a bit much at that point, but I still think the gnome has an unfair option there. It should only be allowed to be a Thief / Mage. Why not be allowed to be Bard, though? Favorite class of the gnome in the 3.5e D&D? Bard. So enough said, let's have a look at my suggestions:
Human (Dual classing)
The human has access to all classes and all that are reasonably at reach to his first are optimal choices for dual classing.
Ranger > Druid or Barbarian
Barbarian > Ranger, Cleric, Druid or Thief
Monk > Fighter or Thief
Bard > Fighter, Cleric, Mage or Thief
Half-Elf (Single and Multiclass)
The Half Elf may not have access to all classes, but he is a master at multiclassing and connecting skills he already has to seek new ones.
Gnome (Single and Multiclass)
Cleric/Mage (not illusionist)
Thief/Mage (not illusionist)
Halfling (Single and Multiclass)
Dwarf (Single and Multiclass)
So far this may be unappreciated but there are fundamental additions that I think should be considered such as the Half-Elf Ranger/Thief, Dwarf Paladin, and Gnome Bard.
Now, on to the NPCs classes fix, I think that there's still a lot of work to do there as well. First, making Tiax a worthwhile character would certainly help enjoy him in the game, but giving NPCs new kits and special abilities would be well appreciated. What makes the most enjoyed characters desirable? They have somethings that others don't and they're essentially members you want to have in your party because they kick some ass. What makes Minsc, for example, so special? His berserker rage even though he is a ranger. Then, his exceptional strenght. But what in this case consists of the most interesting enhancement of the first Baldur's gate characters? In one word, that would be: kits. Giving a kit to all of the basic classes (Khalid's fighter), or at least giving them abilities worth looking at them like they were of a special other class (such as Mazzy's paladin abilities in BG2). Otherwise, it could be one of those above multiclass that no other character may have.
Looking forward with the extension of classes in BG with the importation of all the extra class features from BG2.