[Barrett]

If you were going to compile a list of The Most Effective US Air Units From Then Til Now, you would not include the three most famous:

N.124
The Black Sheep
The Tuskegees

Of those, arguably the one that inflicted the most damage on the enemy was the Black Sheeps (!) but frankly, USMC claims were nearly always way far off the mark because there was no provision for independent review, as in the AAF. (I hasten to add that a lot of other outfits overclaimed considerably.)

The 332nd FG has claimed approximately forever that it had a perfect record on bomber escort, which was demonstrably absurd. However, the Red Tails had an EXCELLENT record in that regard. They're mainly victims of their admirers' zealousness and their own reluctance to correct the errors.

What were the most effective US air units?

Well, I'll open nominations with:

27th Aero Squadron, WW I
USN Bombing Squadron 6, WW II
USN Fighting Squadron 15, WW II
509th Composite Group, WW II (!!!!!)
4th FIW, Korean War
8th TFW, Vietnam.