Braixen. Specifically, the shiny version; it's absolutely ravishing. Some Pokémon I like better before they hit their final form, and Braixen definitely tops the list. I just love how giddy it is! And those slick moves when it's wielding its stick... yowza!
In second (and first if we're talking utility), it'd be Weavile. Fast and deadly! Plus, I've liked Sneasel since the original GS.
Other honorable mentions:
Gallade
Gardevoir (Always liked it; the shiny Mega is awesome)
Absol (Love that Mega.)
Mawile (Its Mega Evolution is a DEATH MACHINE,; I once took out Aegislash and Mega Mewtwo Y with it alone, two on one, with skillful use of Sucker Punch.)
Goodra (I want to name it "Fatty McLovins" but there aren't enough spaces...)
In the past, I've also used Leafeon, Froslass, Gliscor, and Yanmega. Oh, and I like Espeon too.
For mythicals, I've always liked Celebi, just because it's so awesome and adorable. I like Victini too, but I think they butchered its cry in X and Y.