I use Dragonite, though most competitive teams will run either Garchomp or Salamence because they're superior. All three have their uses:
Dragonite can do whatever the other two can, but not as effectively. I hear it makes a good Subpuncher though.
Salamence will either be a CSmence, a DDmence, or a mixmence.
Garchomp... I have no idea off the top of my head if its Sp. Attack is useful, so it'll probably just stick to DDOutrage/ CBoutrage. Also makes a great Tyranitar partner.
A lot of people on GameFAQs use Dragonite out of favouritism (or so I've seen thus far), and personally that's the main reason I use it rather than Garchomp, since my team badly needs a ground-type. On most truly competitive teams though, you're more likely to run into Salamence and Garchomp.