What are the best 6 pokemon to have for Pokemon Diamond?

I've played and restarted my Diamond about 4 times and everytime, I always do HORRIBLE and cannot beat the game, but all my friends have... What pokemon can help me the most?|||Here was my team to beat the Elite four:(all Pokemon level 55)





Infernape


Skunktank


Empeleon


Roserade


Starraptor


Garchomp|||Where have you had the most trouble, my son?|||first, you need a strong fire pokemon like rapidash. and a grass pokemon, like torterra or Roserade. and a ground/water pokemon like empoleon, or etter, quagsire, but his rapidash knos solar beam, and you need a ghost/dark pokemon for the fourth e4, and i used spiritomb.