There are hundreds of possible combinations for your materia setup in Final Fantasy 7, so it's hard to tell you all which ones are the best. Below is a compiled list of some of the better ones that we could think of. If you have one you'd like to add, send an email to the webmaster.
Final Attack + Revive
Anywhere: The most basic combo for your character's dying act - He / she / it will cast the Revive spell on themselves right before dying.
HP Absorb + Knights of the Round
Anywhere: When KoTR attacks the enemy and they die (because they will), you will gain a lot of HP from it.
MP Turbo + Knights of the Round
Anywhere: If your Magic stats is pretty high, this will cause each knight to deal up to 9999 damage. This means 130,000+ damage altogether.