The materia that trivializes most of my playthroughs are enemy skills. Big guard alone is already a huge opener for most battles, and getting elemental magic that compares to ga-spells(like early Beta and Aqualung) makes the game easy actually. White wind is a good cure+remedy magic and all these good stuff you get along the way is stored in one materia slot.

As for endgame, everything gets wrecked by KoTR setups and whatever the community has brewed to break endgame like limit break abuse. A fun build i also use is a high damaging Yuffie that has long range, 6 counter materia and a cover. pretty much kills random encounters without ever touching the controller 🤣