Best team to go with Serperior?

So I just bred myself a Jolly Overgrow Snivy and I'm wondering what team would work well with it as a Serperior.

None.

It wants Contrary and Timid.

To build on with what Dark Azelf said, Serperior is useless without the Contary ability, which reverses all stat changes that happen to it. The main reason why Contrary + Serperior is so good is because Leaf Storm would increase its Special Attack instead of decreasing it.

Serperior's mediocre stats all-round, except for Speed, let it down otherwise, and without Contrary, there's little reason to use it over things like Ferrothorn, Mega Venusaur, and even Mega Sceptile.

And when you do get a Timid Contrary Serperior you basically want to team it up with stuff that beats Talonflame, bulky Grass types (especially Mega Venusaur), Mega Altaria, Unaware Clefable, and Heatran, Scizor+Ferrothorn, or Mega Charizard Y depending on its Hidden Power type, etc. Stuff to remove its checks and counters really. It's also good to have teammates that appreciate what Serp does for them/the team in return (checking offensive Water types, killing Mega Slowbro and Hippowdon, etc).

Well, if you really don't have the option of Contrary available to you (hint: ask in the Trade section of the forum), then you could still make it into some sort of a support mon with Dual Screens+Taunt+Giga Drain. Though, in that case Timid is still the way to go.

Also keep in mind that this set is heavily outclassed by a ton of other supporters like Klefki, Thundurus and Meowstic (hint: higher Priority > high Speed), or anything that gets access to Steath Rock. But if you don't care for high level strategies, then it probably doesn't matter, anyway. :)

As for team partners...you'd need Pokemon that can take advantage of Screens being up, so hard hitting stuff with only mediocre defensive stats are good options.