GM. It's the most obvious, system neutral term around, IMO, so I've had the habit for years of using it regardless of whether or not I'm talking about D&D or some other game.
Heck, at the time I developed that habit, I wouldn't have been caught dead playing D&D anyway, so I think I started using it specifically to avoid using the term DM. But the usage stuck. Even now that we're playing more D&D than anything else, I think it's the best obvious system neutral term to use.
Part of that is that I kinda get tired of dungeons, and I don't want to imply that that's what any game I run is about.