The only other series that ever came within an inch of capturing the same feeling for me was the Close Combat series. But that series has always suffered from too many different design visions and an inconsistancy across the board, while WINSPWW2 and WINSPMBT have had a consistant design team ever since the Windows version was launched. We might fret about the OOB's and TOE's and this and that, at this point the game has been functionally complete for years and years. We're just inventing new reasons to annoy Don and Andy by now, like they need it.
But something this series has always done for me that nothing else has allowed is to capture the entire picture of the war, from the individual man to Regimental level and allow you to explore it in any shape, size or dimension you see fit. Whether its a daring airlanding on Crete in 1941 or Japanese tankers desperately trying to break through in Burma in 1944 or even the Spanish Civil War, you can at least make some attempt to model every concievable conflict in the alloted time span. Virtually every other tactical WW2 wargame I've played, including my beloved Close Combat, only allows for a snapshot. Whether its just of one front or even a single month out of a year.
Nothing else comes close to SP, and until Gary Grigsby himself steps up, Don and Andy will keep my attention for the forseeable future.