Well, considering this is the first time I'm really playing through the game, let's see how I've done so far. . .

Since you've gotta have Cloud pretty much everywhere, he's on the list. I'd say the three people I've used the most outside of Mr. Strife are Aeris, Red XIII and Cid. Of course, I just barely got everyone and I haven't had a chance to really use them all so far (I'm only at the Temple of Ancients). I'm one of the people that tries to use everyone at certain points, unaware if it's beneficial or not. Vincent and Yuffie aren't bad, and neither are Barret, Tifa or Cait Sith. So far, I've usually used the aforementioned three the most.