I still own 2nd and 6th Fleet. Was good to play alone, but I shelved it when I found Tokyo Express, which excelled in delivering solitary playing, which it was made for by design.
Also a fan of the Third World War and Assault series by GDW, I also played Gulf Strike and Flight Leader.
I wish that somebody would have converted Flight Leader into a PC cosim. If I were a coder, I would do that game, most likely. But lastr time I did code, it was in GFA Basic 3.0 - on Amiga.
Ambush! was a good solitary WWII game, too.