Combat Reflexes, hands down. Mainly because it's such a widely-used pre-req for other feats and prestige classes.
For instance: Expert Tactician. This feat is a must-have for monks, since monks can trigger it by:
1. Going first. Monks usually have high Dex, so they often catch foes flat-footed.
2. Stunning Fist attacks. Stun is a great feat, since it makes the bad guy drop his weapons, take a -2 to ac, not act, and lose his dex bonus.
3. Sneaking. Monks have Hide, so they can get a surprise round on Foes.
A monk can potentially:
1. Get a surprise round against a foe, charging (an attack) that also triggers Expert Tactician (another attack)
2. Win Initiative, which triggers Expert Tactician again. The monk then Flurries. One of the Flurry attacks is a Stunning Fist attack that works. Alas, Expert Tactician already was triggered so no followup attack.
3. Opponent is stunned, so he can't act.
4. Monk's turn again. Another 2 flurries.
At first level, best case scenario is 7 attacks in 2 rounds (plus surprise round) with no counter attack. Nice!
-z