1) animee_ already answered that. FFX has a limit of 99999 per hit.

2) Wakka's Attack Reels, if you can line up all the slots correctly. 12 x 99999 = 1199988.

3) I'm sure I probably had someone do lots of damage in FF6 with the Genji Glove + Offering combo, but I don't remember if I did. The highest I remember is Auron doing about 20000 with the Masamune and 1 HP. I'm assuming summons don't count, though. If they do, KoTR did about 65000 for me in total. I've never really bothered über-levelling my characters, so that explains the crappy damage compared to everyone else's.