Yeah as False said, Breloom is my favorite competitive, but Sceptile has a special place in my heart as my favorite Pokemon since I played Pokemon Sapphire. He's a badass gecko with knives on his arms...what else is there to say? I've never played RU before, but I'm sure if I did Sceptile would probably be my #1 competitively as well, since he's such a monster in RU. Competitively though my favorites are Breloom, Heatran, Latias, Reuniclus, and Honchkrow.
I always feel like everyone has a list of favorite Pokemon looks and charm wise, and everyone also has a list of favorites that they like to use competitively. Breloom is amazing, and I've never had a really successful team without him. I personally dislike the SD set, it's too predictable. The Technician Focus Punch set is my personal favorite (I was one of the first ones to use it) because the set plays too many mind games, which is what I love about it. Heatran is also one of the only good Fire types in OU, and combined with its great typing and move pool it can really force a lot of switches. Latias is a bulky attacker, and she's very good in today's weather infested meta game. Reuniclus is a new favorite of mine, and although it's hard to use at times, it's a monster in the hands of an experienced player. Honchkrow is insane in UU, and I sweep teams with Honchkrow in UU almost as much as I sweep teams with Reuniclus in OU. It's the equivalent of Salamence in OU, but instead of speed it packs priority. Great frickin Pokemon.