Which Pokémon in Pokémon X/Y are banned in tournaments?

When playing against buddies during Christmas I want to know which Pokémon are considered "too strong" or "overpowered". In the past generations of Pokémon there have been lists which are banned in tournaments.

I assume the following are:

• Mewtwo
• Xerneas
• Yveltal

But are there others? In that case which might that be?

According to the Smogon X/Y Hub page, Blaziken and Deoxys-N were quickbanned from the Pokémon X/Y Overused (OU) tier, and there are currently discussions around whether or not to ban Gengarite as well (Mega Gengar).

As for the current Smogon ban list, do note that it has been adapted from the Black/White list, and so is subject to change. It's also split into two lists: Pokémon currently available in X/Y, and Pokémon that will become available upon the release of the Pokébank.

Here is the current Smogon tier page, listing restrictions on play as well as Pokémon that are banned. If you're planning to hold a tournament (especially after the release of Pokébank), you should consider banning some or all of these Pokémon :

• Mewtwo
• Xerneas
• Yveltal
• Arceus
• Blaziken
• Darkrai
• Deoxys
• Deoxys-A
• Dialga
• Giratina
• Giratina-O
• Groudon
• Ho-Oh
• Kyogre
• Kyurem-W
• Lugia
• Palkia
• Rayquaza
• Reshiram
• Shaymin-S
• Zekrom