Dewott. There are no other answers.
I mean just
look at him. He's badassery and cuteness rolled him into one dual-wielding lutrine package. Otters are my favorite animals, and Dewott is an amazing otter. He may have no use competitively (seeing as he's not fully evolved and I refuse to let any Dewott I raise ever turn into an ugly sea otter monster), but that hasn't kept me from trying. It's funny watching people's reactions as he tanks thunderbolts when properly raised and equipped. More of a novelty thing, but eh.
If I can talk about the runners up (I hope that's allowed)...
Altaria is by far my favorite dragon and one of my favorite Pokemon. Cute, and fluffy and deadly. The mega evolution introduced in ORAS is absolutely incredible; the fairy/dragon typing lets her be the best at everything while shrugging off most common attack types, and tanking the ones that do hit her for super effective damage. Plus, she's so fluffy!
Gardevoir's another favorite of mine. There's something very regal and elegant about her. Her typing's great, she's good at her job, and her abilities are fun to mess around with. Can't say I'm a big fan of her mega evolution, but oh well.
Finally, Scafty. I can't say why, really, but I just kinda took a liking to him. He's got this mischievous little smile that you can't help but like. And mischievous he is indeed. Even if Gen 6 has made life harder for him, he's probably my favorite Pokemon to use competitively. His trick is to basically get out there and not die. He can boost his attack and defense, use drain punch to heal himself, rest if he needs to fully heal (which he has a good chance of instantly waking up from due to his ability), shed status effects that could cripple him, ruin enemy strategies by knocking away their items, and just generally be awesome.
And I could go on and on and on, but I'll stop here. Those are my four favorites.