Mine is a Gardevoir I have in pokemon Scarlet, she can one-shot almost anything

Probably the level 100 Gardevoir I had when I was nine, it could mega evolve and I used it for a lot of battles. Nine year old me thought it was overpowered because I didn’t know it was weak to steel types, then it got absolutely destroyed by Steven

Ooh, that's a tough one. I'd say my level 100 Cinderace in Sword, but for some reason you said no legendary or "singular" Pokemon allowed, so I'd probably have to go with my Gothitelle for Sword since it and the hacked(?) Mimikyu I got are the only non-legendary or "singular" ones on my team. For Arceus, my alpha lvl. 80 Sylveon I got from a space-time distortion, and for Scarlet, probably my level 100 Veluza. If not, my Tinkaton.

Nah, the Cinderace is allowed

Had a level 100 Lucario in x a few years ago. Took me months.