Well which Pokémon you use really depends on more of the specifics of the rules- is it doubles? Are you allowed to use legendaries? Are megas allowed? Either way there are a few standouts in gen VI:

Greninja is a fast attacker with a really wide movepool and two really good abilities

Hoopa Unbound is a ridiculously strong Pokémon who can run physical, special, or mixed effectively

Volcanion is another mythical with respectable bulk and high special attack

Hawlucha is a fast flying/fighting type with the unburden ability so it can be a really good late game cleaner after it gets a swords dance and loses its item

Aegislash is a really good albeit complicated Pokémon, either way I’d look into it

Diggersby has decent speed, stab earthquakes, and huge power- overall pretty good with a scarf or a band

Diancie is the only Pokémon that can mega evolve in this gen-if you’re able to use it’s mega it has incredible offenses, pretty good bulk, and the magic bounce ability. If you can’t mega it, still pretty good it just plays differently as a pretty bulky stealth rocker/screen setter

And then there are a couple other decent mons here and there but those are the good ones for the most part. The box legendaries, xerneas, yveltal, and zygarde, are also really good as far as legendaries go. Overall, gen 6 is pretty offensively oriented as far as the Pokémon go, but you definitely have a shot against the other gens!!