I can think of 2:
First I would argue the first battle with Mars in Diamond and Pearl. I cannot recall if this fight is different in Platinum, but her team consists of a zubat with toxic and a Purugly at level 16 that is bulky and fast. This is slightly after getting your 1st badge mind you. Purugly has fake out and faint attack, plus an oran berry for slight healing. The moment you challenge it you can get flinched, and evasion boosting or accuracy debuffing won't work thanks to faint attack. This is also making sure things go smoothly because if you're hit with toxic by that zubat and don't have an antidote you're screwed. If you have a shinx then zubat isn't a problem, but if you don't for whatever reason then taking out a zubat shouldn't be a problem. If you started with Chimchar then purugly isn't a problem, otherwise find a strong physical attacker and teach it rock smash.
Second one is mainly for speed running ORAS. I saw a video explaining how in Brawly's gym there's a black belt named Hideki with a single machop that happens to have near perfect IVs (about 26/31 in each stat). He's a mandatory trainer battle too as there is no way to avoid him. He's not OP if you're playing the game like normal, but the video interested me because it explained that his machop having amazing IVs is due to an oversight. It is technically overpowered, but not if you have a team and if you're not a speed runner.