If you haven't played it yet I think
Dreams of Desire has some of the best M/S content out there. It's not just an M/S game, but the relevant parts really are spectacular.
If you don't mind japanese eroge
Itsumama 2 (I think that might be the wrong name but whatever) is really hot too, and it's just about the M/S dynamic. The translation is great.
Shut Up And Dance is also a must play for M/S lovers in my book. It's also not just about M/S, but that relationship takes front and centre stage.
Other than that, Serj got most of my favs already.