A team that worked very well for me was this one:
Night Slash, Surf, Waterfall, and Dark Pulse is what I have right now. There weren't enough good water types throughout the rest of the region I wanted to use, so I used the starter. Besides, he looks pretty cool, too
Thunderbolt, Confusion, Psychic, and Dazzling Gleam. Gardevoir's specialty is special attack, so I don't use any physical moves. It has a very diverse move pool, and can take out a variety of types. It is one of the best members of my team.
Grass Whistle, Magical Leaf, Sludge Bomb, and Facade. I have always loved Roserade a lot, what with the special attack and speed. Its defenses aren't it's selling point, but if you throw into a battle, as long as the other Pokemon isn't strong against it, it can usually take it out before it can get hit. I have Grass Whistle mainly for catching legendaries, and Sludge Bomb/Facade for type coverage, but you can replace any of them with moves better suited to how you play.
Close Combat, Wing Attack, Fly, and Brave Bird is a pretty good move set. Wing attack is powerful and has 35 PP, so having Close Combat and Brave Bird as moves only used in harder fights is okay. But it might be a good idea to replace Brave Bird with Facade, it's always good to have a normal type move. Staraptor is reliable and very easy to fight with.
Flame Burst, Shadow Ball, Curse, and Inferno worked well for me. Flame Burst has 15 PP and high damage, works well as a PP move. Shadow Ball works against many types, and, like Flame Burst, has plenty of PP. Curse is good with tough battles, but since there aren't many of those in X and Y, you can replace it with something else. Inferno is good in longer battles, too. Chandelure has very high special attack, but couldn't pull off a physical attack to save its life. That's okay, though, because both of its types are predominately special. It is pretty late in the game before you can get your hands on a Litwick, though, so I take a Charmander from Professer Sycamore and use that until I can replace it.
Payback, Ice Shard, Screech, Strength. Payback is purely to have a physical dark type move. Options are pretty limited there. If you can find a better one, use that instead, because Weavile's speed is huge, it even outspeeds Greninja, so it is very rare that the damage will ever be doubled there. Ice Shard is very much the same, not many options. Screech is, obviously, to lower the opponent's defense so that the previous two moves work better. Strength is a powerful move with enough PP to get you through battles without a type advantage. You're screwed if Waevile has to battle a fighting type, but that's what Gardevoir/Staraptor are here for. Also, you aren't able to evolve Sneazel until AFTER the end of the game, but Sneazel is perfectly capable of holding its own, even in a team of fully evolved Pokemon. Imagine how it does once it evolves!
There are some Pokemon on this team that level up slowly, but when I played, I grinded to get everything up to level 30 just before Reflecting Cave and proceeded to beat the champion 10 levels above all of her Pokemon. This team has remarkable type coverage, and it made the game relatively easy, but not boring. It also works when fighting against legendaries and trying to catch them. It took about 5 tries to get Mewtwo, but I suck at catching Pokemon. This team wouldn't do any good competitively, but in game, it works like a charm.