Any strong Pokemon really. I'll help ya out here...
1st Elite four Member: Bug
Use: Fire, Rock, or Flying types to win this battle.
2nd Elite Four Member: Ground
Use: Grass, Water, or Ice to take this member down.
3rd Elite Four Member: Fire
Use: Water, Rock, or Ground to hit super effectively.
4th Member: Psychic
Use: Bug, Ghost or Dark types to win and advance to the Champion.
Spiritomb, Milotic, Lucario, Garchomp, Roserade and Togekiss
Go Here to check out what Pokemons they use.