Well, the answer to your inquiry really depends on what manner of gamer you are; are you the old-school, quintessential RPGer or do you lean more toward the 'powergaming-style' side of the spectrum? If you choose the previous, then you should pick whatever HLA (High Level Ability) that best suit your interpretation of the given character. If you, say, envision Keldorn more as a spiritual warrior rather than a 'I-bash-you-to-goo' sort of zealot, you might want to give him the Summon Deva special ability to portray his devotion RP-wise. If, however, you want your characters to excell combat-wise, here is a little out-of-the-hat list as to what sort of HLAs you ought to choose:
For Minsch, Keldorn, Jaheira and other Fighter-like characters, consider these two HLAs: Critical Strike and Whirlwind/Greater Whirlwind. If your Fighter is equipped with a two-handed weapon, GW is a must-have. If, however, you prefer to dual-wind, GW is more or less rendered superfluent as you can easily access 10 hits per round with certain weapons and the spell, Improved Haste. In that case, choose CS, meaning you will be able to perform 10 attacks per round that (and this is the part where you are supposed to drool, hehe) NEVER miss.
For your swashbuckler, do consider the UAI (Use Any Item) Special Ability. Self-explanatorily enough, it means you will essentially be able to use whatever item you pick up, including but not limited to class-specific items. HL Traps are also devastatingly useful, albeit somewhat cheesy.
For mages, the Comet spell can be quite handy. It is a party friendly Area of Effect spell which does decent damage and knocks enemies down, buying some time if such is needed. Dragon's Breath is party friendly as well, but you will note that many enemies are fairly resistant to fire, so I suggest you consider Comet, instead.
Summon Deva is a decent HLA for Paladins, but if you have a Cleric in your party (you mentioned Aerie, did you not?), leave it up to her and focus on the more combat oriented HLAs, instead.
Hope that helps,
CSN
