Alright, well I'm going to hopefully start an interesting conversation here, as I see the lower rankings are in need of some work. A long time ago, when I was pretty new to the site, I enjoyed using Flygon. I told someone that I used it in OU, and they said, "Go ahead and nominate it on the viability rankings. I dare you."
Months later, I finally have followed up on that. I've come back with more experience not just in the OU metagame, but on the forums, and I feel I deserve to at least have a say. I'm hoping not to get chewed out for this, but we'll see. Let's do this.
Flygon: Unranked ----> C+/C
The main reason this person told me Flygon was bad was because of Garchomp. This Generation, Garchomp received a Mega, leaving Flygon in the dust. However, I feel Flygon at least has some use in the metagame.
Advantages over Garchomp:
•Huge move pool. Garchomp kind of has a 4 move slot syndrome and is fairly predictable.
•Access to U-Turn as well as many support moves
•Access to Roost and Defog (Will get more into that)
Disadvantages:
•Much less immediate power.
•That's... Actually about it. Flygon stats are a little lower, but not low enough where they are useless.
Moving on to its new purpose in X&Y: Defog support. Flygon has Levitate, making it immune to Spikes, and it's typing is resistant to Stealth Rocks, unlike Mandibuzz or Latios. It might not be as bulky as some common Defoggers, but an advantage over Latios is that it doesn't have to get Pursuit-trapped every time Bisharp comes in. Just predict the switch and Earthquake away. A quick set I threw together:
Flygon @ Leftovers
EV: 252 HP, 252 Def, 4 SpE/Atk
Ability: Levitate
Impish Nature
-Earthquake
-Roost
-Defog
-U-Turn/Dragon Claw/Tailwind
This set takes full advantage of Flygon's excellent support move pool. Tailwind is especially helpful with slower sweepers, but you can choose to run U-Turn over it for a bit of momentum. A specially-defensive set can be used as well.
Other options:
-Fast enough to pull off Taunt sucessfully
-A Scarf set is also viable with U-Turn
I'm hoping this is enough to at least start a conversation. I haven't been too involved in the viability rankings just because of my unique opinions. I feel people would argue way too much with me. If any more information is required, feel free to let me know.