Who to pick, and who to drop, depends on your playing style.
For example, do you use spells as your primary offensive,and fighters to mop up? Or fighters to hack n' slash, with spells to protect and give support fire?
My style is the latter, and I used this party in SoA:
Kensai - NG
Minsc
Aerie
Jaheira
Keldorn
Jan (Imoen from Chap 4 onwards).
In ToB, I dropped Minsc for Saraevok.
Try to have a balanced party - tanks, offensive spell casters, defensive spell casters, and a thief (to find and disarm traps, and pick locks). My party has all these elements and I like to use Imoen (w/Staff of Magi) as a scout to go ahead of the party (invisibled, of course), to check out the next area before I move in with the rest of the party.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
