Diamond is on 3DS, and there are other games on this one : White / Black and White 2 / Black 2. I played Black 2 myself and I highly recommend you try that one (or White 2)!
They have what I think are the best means to train full teams (top level trainers after the League, lots of Rare Candies, a Galleria you can devote entirely to raise pokémon's happiness, an underwater treasure for which a NPC will give you more than one billion pokédollars (it sure helps when one wants to fill their team with lots of vitamins ^^), and the White Tree / Black Tower in which you can fight trainers in a wide range of levels - the higher the stairs, the higher the level - ; once you have reached one of the stairs, you can leave it whenever you want to come back in right away to refresh it, no limit or delay! Each time you come in, the trainers are randomly generated with various pokémon!
I really think all of this puts Black 2 / White 2 among the best of all Pokémon games. ^^