I can't believe how many people have posted saying Ultimecia is hard to beat! Surely you just use aura and then use Squall's final limit break which incurs over 100k damage? (a lot more than that actually, I think, with his ultimate weapon).
Final Fantasy 7 - Emerald Weapon I found really hard. Only beat it on my 3rd time round playing the game, and having worked for many yellow counter materia + limit break ...
Ruby Weapon I never did end up defeating, for one I found it too hard, and the reward is pretty crap too, a Gold Chocobo with poor stats?