Most of the Level 3 and 4 Limit Breaks. Meteor Rain and Doom of the Living are boss killers by themselves, there is really no point in describing Omnislash and Catastrophe. Not to mention if you get Great Gospel for Aerith, the rest of Disc 1 is made incredibly easy. Its nice that the developers made it really difficult to obtain without a guide or hindsight.

Most of the Ultimate Weapons, Cloud may already be a damage dealing wonder, but when a simple weapon can make physically weak Yuffie start doing consistent damage on par with Cloud, you know its a game breaker.

Several of the Enemy Skill materia. Beta has already been mentioned, though I feel Trine, Waterlung and Magic Breath are up there as well. Yet, I feel Big Guard and White Wind are the biggest "I win" abilities. Big Guard casts Wall and Haste on your whole party for a fraction of the cost and resources needed to set it up with Magic Materia and All Materia. White Wind restores health in the amount of the casters health. Considering you have HP up Materia, its not difficult to get 6000+ hp restoration early, not to mention its has a secondary Esuna affect restoring your status. Its basically a weakened Pulse of Life Limit Break that can be used by anyone and everyone. The really scary thing here, is that both spells are easy to obtain in Disc 1 because they belong to common enemies on the map and both are available before reaching Barret's hometown. You get Manipulate materia a little later and then its just a ten minute backtrack to get the two spells that will free up quite a few Materia slots and help with all defense for the rest of the game.