Which Pokemon do you think is just ALWAYS THERE in an opponent's team that makes you throw up over boredom and annoyance?
You forgot to put Lucario on this list. It's so OU it's not even funny. Not that Lucario are bad. I, like, half of competitive battlers, use one (or two).
I guess the fact that he's OU combined with him being the Fire-type starter (which is usually very popular anyway) means that everyone loves him.
Garchomp is another one, I mean, sure it's got a bad ass move set pool, killer stats, and not to mention a cool design, but do you really need a Garchomp in every team, why not just use a Flygon? Those are just as good and along with Levitate... It's like beating a dead frickin horse. Just use Flygon.
The only real advantage Flygon has over Garchomp is Levitate, and Garchomp's Sand Veil is hardly a bad ability.
Flygon has Roost, no other Dragon (Or Roost user for that) can still be immune to Earthquake after using Roost, Garchomp has no way to recover HP (baring the Uber Lati twins, which already have Recover on top, so really, they don't need Roost).
Screech compliments his moveset nicely, You stay in, you're going to get hurt, you switch, the next Pokemon will get hit hard.
Tsk, We also mustn't forget that Flygon has access to Fire Punch, which is Stronger and more reliable than Fire Fang. And the fact that the majority of his moves don't have to be bred as well means Migrating a Flygon very easy to do and get a moveset on with Fire Funch.
Also, no other OU Sweeper can boast this, Flygon takes reduced or no damage at all from Electric, Fire, and Ground, all of which are popular in OU.
On top, he resists Stealth Rock, is Immune to Spikes, and takes no damage from Sandstorm, and we all know Garchomp will take damage from Spikes, while Flygon doesn't.