Ok, My answer is "brocks strongest Pokemon in the game" someone else can do the anime one cuz I'm not good at anime.
- Gen 1. Onix
- Gen 2. Onix
- Gen 3. Onix
- Gen 4. Onix
- Gen 4 (Rematch). Onix
- Gen 5. (PWO) All Pokemon are in the same level
So the answer is Onix. I guess Onix is the strongest of brocks Pokemon in the anime too but I'm not sure since the Pokemon in the anime has no level.