If you mean for competitive, then Adamant, no question. Most of the things that outspeed it in its tier don't carry Rock, Ground or Water moves, so it can take a hit from them, especially with Intimitdate. And many of them are frail enough to be taken out by ExtremeSpeed. And even with neutral Speed, it still outspeeds most of the bulkier Pokemon it wants to hit with Flare Blitz and Close Combat.
If you mean generally, then still Adamant. The value of ExtremeSpeed coming from a high Attack stat can't be overstated. --- "It was like unwrapping a wonderful present -- a present made out of presents." - Graham Linehan, describing the Legend of Zelda: Majora's Mask