Adamant and Jolly are the contenders. Which to use depends on what you want to do with the Flygon and what benchmarks you want to hit.
I recommend doing some damage calculations to gauge how much the 10% boost in Attack will matter in the format you're playing. For example, if you're playing Gen 5 UU, it might bother you that Scrafty (for example) survives +1 Jolly Outrage:
+1 252 Atk Life Orb Flygon Outrage vs. 252 HP / 0 Def Scrafty: 282-333 (84.4 - 99.7%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ Atk Life Orb Flygon Outrage vs. 252 HP / 0 Def Scrafty: 309-367 (92.5 - 109.8%) -- 56.3% chance to OHKO
...But if you can't find such examples, then Jolly is a very safe bet. Base 100 Speed is an extremely crowded speed tier (and so is 90+ beneath it) -- you'll want to keep your nose in front, especially if you're running Scarf Flygon for speed control. I would be most likely to use Adamant on the DD set.
