In the entire Kingdom Hearts series, my favorite summon is a tough choice between Tinkerbell and Cloud. I guess Tink wins for overall usefulness. Constant healing, a revival, and you get to keep your party members!
I love Cloud, maybe because I'm a Final Fantasy VII fanatic...
but either way
the damage that Cloud produced was great.
I also liked Tinkerbell, but I didn't really get hurt enough for her to be useful.