what are the best pokemon in pokemon x/y to use?

Relicanth with Leftovers OR Grumpig with Rockyhelm.

A wide variety of types and movesets with a wide variety of abilities to counter any trainer, one pokemon i have come across is Honedge, this bad boy is amazing once evolved into an Aegislash. You can catch it in tall grass on route 6, it evolves at level 35 into doublade and then it needs a dusk stone to evolve into Aegislash. Before i babble on here is the Pokemon:http://www.serebii.net/pokedex-xy/681.shtml

Just remember to have a Pokemon with move sets that cover it's weakness and a wide variety of other Pokemon's weaknesses, e.g. Tyranitar with: Earthquake, Blizzard, Thunder and Dark Pulse. I could name types that a pokemon like that could cover but i guess you get the drift.

The best ones are really legendaries, try to use pokemon at the top of their stats and EVs e.g.:

Chandelure: Specialises in Spec Atk, this would cause moves like fire blast to wreck havoc

Aggron: Specialises in Def, this would allow it to tank out weaknesses and slowly take others down

Jolteon/Ninjask: Specialieses in Speed, it will always move first if EV trained, ALWAYS

Blissey: Specialises in HP, this pokemon can max up to near 800HP! double the average lv100

Dragalge: Specialises in Spec Def, this would tank moves like fire blast with ease

Arcanine: Specialises in Atk, this would add extra damage to same typed moves as the user

There are so many more but that is for you to choose.

Squirtle, Chespin, Aegislash, Gogoat, Pangoro, Scizor, etc.

There are so many pokemon, so many endless teams!