Edited: I wrote that up based on the Platinum Pokemon League, and I just realized that with Dialga you're using Diamond. My mistake, but most of what is below still applies. The only differences are that Aaron uses Dustox and Beautifly instead of Yanmega and Scizor; Bertha uses Quagsire and Sudowoodo instead of Gliscor and Rhyperior; Flint uses Steelix, Lopunny, and Drifblim instead of Houndoom, Flareon, and Magmortar; Lucian uses Girafarig and Medicham instead of Espeon and Gallade; and Cynthia uses Gastrodon instead of Togekiss.
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Riolu is pretty much too low to be considered (unless you want to go through a lot of work and hassle to level it up), and Ponyta is also too low (again, unless you want to go to more work to get it leveled up to par). Based on that, I think of those you should go with Dialga, Empoleon, Uxie, Gyarados, Mesprit, and Azelf, but I highly recommend training for a while in Victory Road to level up as much as you can stand. I selected the legendaries over your Kadabra, but that's up for speculation. It would depend on if you've taught TM's to your Kadabra or gone to great lengths to train it. As with the other two mentioned, you'd have to level it more to get it up to par first. Gyarados could also use more leveling before beginning your Pokemon League conquest.
What Pokemon you use for each Pokemon League trainer depends entirely on the move sets you have for each Pokemon.
Elite Four 1: Aaron, the Bug-type Pokemon trainer - Yanmega, Scizor, Vespiquen, Heracross, Drapion
If you have any Fire, Flying, or Rock moves trained to your Pokemon, send those Pokemon out first. Ice and Electric moves are also handy against Yanmega and Vespiquen. Scizor is only weak to Fire moves, and Drapion is only weak to Ground moves. Heracross is a Bug-Fighting Pokemon, and as a result, is also weak to Psychic attacks. Be careful using Psychic Pokemon against all but Heracross and Drapion because Psychic types are weak to Bug types.
Elite Four 2: Bertha, the Ground-type Pokemon trainer - Whiscash, Gliscor, Golem, Rhyperior, Hippowdon
If you have a Pokemon with Grass-type moves, send it out first, as that is the only type of attacks that Whiscash is weak to. After that, use your Empoleon's and Gyarados's Water and Ice type attacks for her remaining Pokemon.
Elite Four 3: Flint, the Fire-type Pokemon trainer - Houndoom, Flareon, Rapidash, Infernape, Magmortar
Houndoom will rip apart your Psychic Pokemon, so don't send any of them out against it. Your Gyarados is once again your strongest Pokemon here; use its Water-type attacks to take down his team. Empoleon is also useful, but being part Steel it won't be as strong as Gyarados against Fire-types.
Elite Four 4: Lucian, the Psychic-type Pokemon trainer - Mr. Mime, Espeon, Bronzong, Alakazam, Gallade
You'll somewhat be able to use your Psychic Pokemon as an advantage here, assuming they know moves other than Psychic-type attacks, because Lucian's Psychic attacks will be weak against them; however, watch out for any of his Pokemon that have Shadow Ball or other Ghost- or Dark-type attacks. Use any Bug-, Ghost-, or Dark-type attacks that your Pokemon know to your advantage against his Mr. Mime, Espeon, and Alakazam. Bronzong is only weak to Fire attacks (assuming it has Levitate), but Electric, Water, Fighting, Bug, Ghost, and Dark attacks will do normal damage to it. Flying- and Ghost-type moves are good against Gallade.
Champion: Cynthia - Spiritomb, Roserade, Togekiss, Lucario, Milotic, Garchomp
Spiritomb isn't weak to any type of move, but it's strong against Psychic-types. Use Dialga, Empoleon, or Gyarados against it to start off with. Roserade is weak to Psychic attacks, so use one of your Psychic Pokemon against it. Togekiss is weak to Electric, Ice, and Rock attacks; if one of more of your Pokemon have a move of those types, use them. Lucario is weak to Ground-, Fire-, and Fighting-type attacks and will take normal damage from Water, Electric, Flying, and Psychic moves. Milotic is weak to Electric- and Grass-type attacks. Garchomp is only weak to Ice and Dragon types; if Dialga has a strong Dragon-type move, it would be a good choice here.
Hope that helps, and good luck!
