Rinoa's Invincible Moon or Shooting Star are extremely powerful.
Irvine's AP Ammo always does 9999 damage at high levels.
Squall's Lionheart does around 70,000 in total.
Zell's Duel starts off really good but gets weaker towards the end.
Quistis' limit breaks have their uses, but are generally poor.
Seifer can use No Mercy at 86% and does 9999 to all monsters at high levels.
Edea's [forgets name] is the worst limit in the game.
Laguna's Desperado is similar to Seifer's in damage.
Kiros' Blood Pain (IIR the name) gets powerful at high levels.
Ward's Massive Anchor is almost as bad as Edea's.
SELPHIE has the best limit - THE END. Using THE END will kill any creature instantly. Including Ultimecia. It's rare, but incredibly powerful.