I've found a nice little overview including difficulty, depending on your counters you could do all T4's + Rayquaza.
Note 1: Aggron is harder than Rayquaza
Note 2: Don't forget to exploit double weaknesses
Edit: Absol is by far the weakest of them, start with it if you are unsure and don't want to risk wasting passes (+ Pokebattler or the new Go Battle Sim can help you to figure out the strength of your Pokémon)