Kuja permanently eradicated Terra using Ultima. Sephiroth's Super Nova summon "destroyed" the same damned planets every time he summoned it. None of them were permanently wiped out, therefore Super Nova's planet destroying capabilities were nothing more than cinematic flair. Kuja wins in that argument, no contest.

Kuja wins period. The only reason Sephiroth was even alive in FF7 is because when Cloud tossed him off the edge of that mako reactor bridge, he just so happened to end up encased in Materia. Had circumstances not been just right, Sephiroth would have been found himself dead at the hands of an "inferior" being.

Kuja actively outwitted and outpowered Zidane and his party throughout the entire game. Not once did the heros manage to best Kuja until the final encounter. He's the better villain, hands down.