Rikku, Auron, and Tidus are my usual party, because their overdrives are the best and Rikku is the best at mugging enemies. If I need magic I almost always bring in Yuna, because her mid-level black magic spells cause 3x the damage that Lulu's high level ones do for some reason, not to mention the whole Aeon/healing thing. Wakka is also pretty useful, but I hardly ever use Kimhari or Lulu.