Depends on your party.
Are you an evil type? Edwin then
Good type?
Nalia and Imoen are about tied (with a slight edge to Imoen).
Do you like a pure mage or do you want a versital one?
Total pure would be Edwin, most useful would be Jan
Small comparisoin:
Imoen-
Str 9 (no biggie plenty of items to raise it)
Dex 18 (rocks!! she can hit and keep from being hit)
Con 16 (decent)
Int 17 (dang this hurts her, never thought being so close would make so much more diffrence)
Wis 11 (big whoop for a mage/thief)
Char 16 (she's cute)
AC 6
Nalia
Str 14 (hey she can carry more)
Dex 18 (another good hitter)
Con 16 (decent)
Int 17 (same boat as Imoen)
Wis 9 (hmmm, well she is trying just misguided)
Char 13 (another cutie down home girl from the sticks)
AC 6
Jan
Str 9 (needs some items to help)
Dex 17 (dang that hurts but can be helped)
Con 15 (not bad)
Int 16 (hurts on this, no items raise int)
Wis 14 (wierd for a natural scallywag to have it this hi eh?)
Char 10 (no biggie)
Ac 6
Edwin
Str 10 (not bad for a mage)
Dex 10 (he sucks with out a spell unless you get some dex gloves but those should go to a real fighter or thief)
Con 16 (not bad at all)
Int 18 (his biggiest advantage)
Wis 10 (no biggie)
Cha 10 (no biggie again)
Ac 10 (yep he is a target)
I left of the hitpoints as that can very from game to game alot.
Edwin had 18 intel, but with level cap in, he still won't be able to cast 9th spells, but it helps with spell sucess and memorizing.
His attitude sucks, and he can't take a hit, and he is very one dimensional, basically you want him for his spells. Plus he can't use the katana's to get extra spells.
Nalia and Imoen can use all the mage items, and the mage extra spell weapons, and they can pop up with bows or daggers and hit from afar and stay out of trouble. They also are good trap finders and lockpickers.
Jan is Mr. Versital, and he can make you a ton of money pickpocketing.
All up to you on what you want. My personal favorite is Imoen.
Elminster makes an appearance in the game, so I guess he is the most powerful in it.
All time most best mage is Sage from Groo
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Drink and be merry
Jimbo
"Renegade, Rebel, and Rogue"