So this is what the metagame would look like:
Mainly: SP
Few: Gyarados
Little: Vilegar + Rogue
There's a couple options for you to take (without sacrificing huge consistency from your deck choice).
Dialga/Chomp does excellent against Vilegar and Rogue, and if you tech for anti-SP, then it's certainly a viable choice.
Luxray/Garchomp does excellent against Gyarados, and if you tech it for anti-SP, then it's certainly a viable choice. Luxray has the advantage of being able to tech better, however. You can easily fit in a 1-1 Dialga G Lv.X for VileGar (be sure to play plenty of Power Spray!) or a 1-1 Blaziken FB Lv.X for certain decks like Dialga, Steelix, or Scizor.
Blaziken/Luxray works similarly as well. It can go toe to toe with Gyarados when you go aggro Luxray, and it can also put a lot of pressure on Vilegar by bringing it up each turn and getting around Fainting Spell with possible Burn.
Some great anti-SP techs are Ambipom G, Dragonite FB, Toxicroak G Promo and even lesser played ones like Honckrow SV.
Even though your meta sounds mostly SP, I would tend to avoid playing Machamp/Donphan and similar decks (like Flygon/Donphan). They may have a slightly better matchup against SP, but they have a horrible matchup against Gyarados (take it from my friend who has played both and never beaten a Gyarados). Vilegar is also troubling for both, I would assume. SP is the safer, all around good deck choice because it doesn't overtly sacrifice other matchups.
dmaster out.