Keith Laumer rates high with me. He was a master of funny adventure SF.
Dean Ing also produced some excellent stuff.
Both Ing and Laumer wrote about things they not only knew of but had a real feel for as well. Laumer was in the USAAF in WW2 and went on to serve in military diplomatic functions. Ing was a 1950's USAF interceptor crew chief.
If you are talking about Alternate History as SF, Robert Conroy has some excellent books, covering mot only WW2 but other conflict periods as well.
And if you have not read Taylor Anderson's Destroyermen alternate history series you ought to give it a try
James P. Hogan was sort of a later day Arthur C Clarke and had some great novels and stories.
S.M. Stirling is one of my absolute favorites, although I don't really care about his fantasy stuff.
Probably the top guy, aside from Heinlien, is Jerry Pournelle; they were friends.