Jaheira. She has good stats. She can take any role; pure caster, tank, dps - anything but ranged.
Except for the following:
In BG2 alone (no Throne of Bhaal installed), she never gets 7th level spells, as she needs 3 million XP and the XP cap is only 2.95 million.
With Throne of Bhaal installed, she still gains druid levels very slowly after 1.5 million XP; 14th level (and 7th level spells) are not learned until 3 million XP, and the next druid level doesn't come until 6 million XP (ToB XP cap is 8 million, but I hear that, on a typical playthrough, the party ends up with only around 6 million). This means that Jaheira will not get as many HLAs as most characters (until really high XP totals), and she will be stuck with only 1 7th level spell for a very long time (at 6 million XP, she jumps to 6 7th level spells).