If we're talking 1941 for the US, for AH fighters, it would be the P-40B, P-40E, and F4F-4.
If we're talking 1942, it would be those and the P-38G, F4U-1, F4U-1C, and F4U-1D.
You could probably add the P-47D-11 to the 1941 time period if you are willing have it as a substitute for the P-47B. The P-47B and P-47D-11 both have 2000 HP military power and performed about the same without WEP. The P-47B has no WEP, and the P-47D-11 has 2300 HP with WEP. The P-47D-11 also has a drop tank available, which the P-47B did not have.
Here are dates of first delivery of US fighters, from America's Hundred Thousand, by Dean. Note that these are dates of first delivery to the military, not date of first flight of the model.
P-38, June, 1941
P-38D, July, 1941
P-38E, Oct., 1941
P-38F, Feb., 1942
P-38G, June, 1942
P-38H, March, 1943
P-38J, Sept., 1943
P-38L, June, 1944
P-40, June, 1940
P-40A, Sept., 1940
P-40B, Jan., 1941
P-40C, March, 1941
P-40G, Aug., 1941
P-40D, July, 1941
P-40E, Aug., 1941
P-40F, Jan., 1942
P-40K, May, 1942
P-40L, Jan., 1943
P-40M, March, 1943
P-40N, March, 1943
P-47B, Dec., 1941
P-47C-5, Sept., 1942
P-47D-5, Feb., 1943
P-47D-28, April, 1944
P-47N, Sept., 1944
P-51, July, 1942
P-51A, March, 1943
P-51B, June-July, 1943
P-51C, Aug., 1943
P-51D, Jan., 1944
F4F-3, Aug., 1940
F4F-4, Dec., 1941
F4F-7, Jan., 1942
FM-1, Aug., 1942
FM-2, Sept., 1943
F4U-1, Oct., 1942
F4U-1C, Nov., 1942
F4U-1D, Nov., 1942
F4U-4, Dec., 1944
F6F-3, Dec., 1942
F6F-5, Apr., 1944