I have several. I have a hard time picking favorites, you see. But some of them are:
-Zekrom w/Life Orb or something
Nature: Quiet
Ability: Teravolt
EVs: 104 HP, 148 Attack, 252 Sp.Attack, 4 Speed
Moves: Bolt Strike, Dragon Claw, Draco Meteor, Focus Blast
As you probably guessed, I loveses my Zekrom. It's the one I got when I played my White, and its always been special to me (I guess the story line of Black/White helps a bit with that). And I finally get to use it in battles in X/Y! All shall know the might of mixed attacker Zekrom!
-Vaporeon w/Lum Berry
Nature: Bold
Ability: Water Absorb
EVs: 252 HP, 252 Defense, 4 Sp.Attack
Moves: Scald, Toxic, Wish, Protect
My favorite Eeveelution. This set is beautiful, and has seen me through a lot of battles. I've even taken out 4-5 Pokemon on a single team with Toxic+Wish+Protect.
-Scarlet (Volcarona) w/Life Orb
Nature: Modest
Ability: Flame Body
EVs: 252 Sp.Attack, 252 Speed, 4 Defense
Moves: Fiery Dance, Quiver Dance, Bug Buzz, Giga Drain
Volcarona is a Pokemon that at first I didn't care for, funnily enough. Never cared much for Bug types, so I wrote it off initially. Then one day I used one, and I've loved it ever since. She has her issues (them rocks and Heatran), but she's also the only Pokemon I've ever ever had single-handedly sweep an entire team with. And one of the very few Pokemon I've ever honored with a nickname.
-Togekiss w/Leftovers
Nature: Bold
Ability: Serene Grace
EVs: 252 HP, 248 Defense, 8 Sp.Defense
Moves: Air Slash, Roost, Dazzling Gleam, Thunder Wave
Oh paraflinch Togekiss, you are so awesome.
-Lucario w/Lucarionite
Nature: Hasty
Ability: Adaptability
EVs: 252 Sp.Attack, 252 Speed, 4 HP
Moves: Aura Sphere, Flash Cannon, Dark Pulse, Extreme Speed
This is the one that was hanging around Korrina before it decided to join you. I was a bit...what's the word I'm looking for...touched? moved? that it wanted to join me so bad. It's a bit hard to not get attached to a Pokémon that is attached to you as well. And it comes with a good nature and perfect Speed IVs.
/rant