Pokemon whose levels can be raised to match the player's, like the ones in Battle Towers, don't count. This is what I think I know already. RBY: rival GSCHGSS: some guy on a big mountain; forgot his name and which Pokemon he had RSE: Steven DPPt: rival BW: Alder? Please don't question my curiosity.
In RBY, it's your rival with a Level 65 starter/eeveelution. In GSC, it's Pokemon Trainer Red with a Level 81 Pikachu. In RSE, it's Steven with a Level 58/78 Metagross for Ruby/Sapphire and Emerald respectively. In FRLG, it's your rival with a Level 75 starter. In DPPt, it's your rival with a Level 74/85 starter for Diamond/Pearl and Platinum respectively. In HGSS, it's Pokemon Trainer Red with a Level 88 Pikachu. In Black and White, it's Cynthia, Alder, and Game Freak Morimoto with a Level 77 Garchomp/Volcarona/Zebstrika respectively. In Black and White 2, it's Benga, Janna, and Jariel with various Level 80 Pokemon. In X and Y, it's Diantha with various Level 80 Pokemon. In ORAS, it's Wally with a Level 81 Gallade. In SM/USUM, it's Pokemon Trainer Red with a Level 70/74 Pikachu for Sun/Moon and Ultra Sun/Ultra Moon respectively. In LGPE, it's Pokemon Trainer Red with various Level 85 Pokemon. In SWSH, it's Hop with various Level 70 Pokemon. In SWSH (DLC), it's Mustard and Leon with various Level 80 Pokemon. In BDSP, it's Cynthia with a Level 88 Garchomp. In LA, it's Volo with a Level 70 Giratina. In SV, it's Sada/Turo and Nemona with a Level 72 Koraidon/Miraidon/starter respectively.
Hope this helped!
Rby Rival Frlg rival Gschgss red Rse oras steven Dppt rival Bw alder Bw2 n Xy Serena Sm red Usum red took me an hour to compile up correct me if one is wrong