I'm replaying 7 at the mo and i'm using Cait Sith as i've never bothered with him before, got to Wutai today and i loved this quote:
"Listen you stupid, slimy, sleazy old scab bag! If you want to get women, there is such a thing as the RIGHT way! "
Cait Sith to Don Corneo

I think its the use of the expression "scab bag" I especially enjoy. Also the bit before Yuffie nicks your materia, Cait Sith slipped up and said something about not being one to talk, I haven't done the bit where you find out about him yet, silly thing