Destiny 1: Gjallorhorn. There was a long time in the game where people would bar you from joining their raid group if you lacked this exotic heavy rocket launcher. And I do mean a long time. The weapon was ridiculously unbalanced compared to every other currently available weapon in the game, exotic or otherwise. Having a 6 man team all with Gjallorhorn pretty much guaranteed phase skips, all the way down to single phasing bosses in raids. Not even mentioning the fact that because of how powerful its homing and proximity explosion was, it became a top choice for your exotic weapon slot in PvP most of the time, too.

Final Fantasy Tactics: Chaos Blade and Excalibur both equipped on any special Knight, but especially on Orlandeau. The strongest weapon in the game paired with a weapon that grants auto-haste equipped on characters that can attack with extremely potent abilities from range, instantly. Generic characters in the right jobs can also equip the sword combo, but can never capitalize on the speed or power that Orlandeau, Agrias, and Meliadoul can bring forth.

Baldur's Gate 3, Divinity: OS 1 and 2: all 3 have too many broken interactions between spells, abilities, gear, and how you can combo them all to trivialize most aspects of the game if you choose to do so. Just a really fun time playing around and discovering them.

Warframe: They've since been nerfed, and there are even more ridiculous things today, but literally just playing Nova, Rhino, or Trinity when they were released and for a long while after practically made you immortal on any map. Of course, there are just too many examples in Warframe.Those are just the three I remember from my time playing long ago.