I agree with many of the selections previously mentioned.
However my "favorite" WWII movie (certainly not the "best") is God Is My Co-Pilot starring Dennis Morgan, Raymond Massey, and Alan Hale Sr.
The Flying Tigers (AVG) have always been of particular interest to me. I have done lots of reading on the subject. This dramatization based on the book by Robert L. Scott uses the names of the actual AVG pilots that served in China, and contains some events that actually happened, as opposed to many fictional accounts of the AVG's exploits (like "The Flying Tigers" starring John Wayne). The flying sequences with the P-40's are also well done.
Link to Wikipedia article:
http://en.wikipedia.org/wiki/God_Is_My_Co-Pilot_(film)