How do you catch the best Pokemon on Pokemon Diamond?

Answer 1

I myself am an avid Pokemon diamond player.

The pokemon get better and better the different places you go.

Around twinleaf town and in that area, pokemon are levels 2-10

The farther east you go, the better and better the wild pokemon are.

Once you get to Sunyshore City going up to the elite our, wild pokemon are level forty-ish. Once you get to the fight area (that big island place in the top right of the map) and you get the national pokedex, all the pokemon are level 50-60 and are from many different cames. That is where you can catch the best pokemon.

If you are talking singular "the best pokemon ever", you can find legendary pokemon in Stark Mountain, Snowpoint Temple, and other sites of legendary pokemon.