It depends on what you have available and who you are fighting. Attack Form Deoxys has the highest bast sp. att in the game, and is faster than everyone but Ninjask and other Deoxys. Dialga, Darkrai, and Heatran are also incredible specialists. If you don't want a legendary, there is always Alakazam and Gengar (both fast and hard hitting specialists who work better against different opponents).
my team: Infernape lv. 64-sweeper-starter Garchomp lv.62-sweeper-secret tunnel under bike road Umbreon lv.60-tactical-evee evolved Lucario lv.61-special sweeper-Iron island from Riley Hippowdon lv.59-wall-battle area Staraptor lv.59-sweeper-Twinleaf town
Diamond is not a Pokemon but i think Arceus is one of the strongest Pokemon ever.
the best Pokemon on diamond would be arceus but you can only get that from action replay, other than that i think the best Pokemon would be giritina..
the best Pokemon for the Pokemon league is definetly latios
Palkia
the best Pokemon in diamond is arceus, but you have to use action replay or wi-fi mystery gift
No Arceus is the best Pokemon in Pokémon Diamond.
The best Pokemon on Pokemon diamond is obviously a Bidoof...Joke :)No this is entirley an opinion based question but the best aka most powerful,and most defensive is Diagla
Chimchar
a lot of legendary Pokemon
swampert miltic
in my opinion Pokemon diamond is the best; i have both games but i don't play them.i have played both of them before, but diamond is the best- and its my birth stone =)