What is the best type of Pokemon to challenge the Elite 4 with in Pokemon Diamond?

It is good to have a variety of types. I suggest strongly to have a powerful water type, a powerful fire type, a powerful grass type, a good ice type, a very powerful normal type, and last, but not least by any means, it helps to have a Luxray, at least level 72. Also, if you want to survive, bring about 15-20 revive, and just in case, after you have defeated someone, save your game.

I would say if you've got the national dex on 1 of the Pokemon games (emerald, ruby, sapphire or diamond) get a dratini as dragonite is a very powerful Pokemon and trade it to pearl. My team would be: Infernape Lvl 55, Dragonite Lvl 59, abomasnow Lvl 57, Gyarados Lvl 60, Drifblim Lvl 62 and Pikachu/Riachu Lvl 58 (that knows volt tackle)

To get volt tackle breed two a mix of pichus family (pichu pikachu and riachu) and the DAD holding the light ball. The pichu will know volt tackle.

I also think that you should have two contrasting Pokemon egg. Fire - Water, Fire-Grass, Bug-Fire. This way you can cover a large range of enemies.