For me D&D (and it's ilk like C&C, etc.) are good "common ground" games. If a group can't figure out what to play or you can't figure out what game to run to get players, D&D will usually fill the bill as most people have either heard of it or played it and are willing to give it a go to be able to play something. And it's fun, too.
However, Savage Worlds is beyond a doubt my favorite RPG by a lot. A lot a lot. I like it's flexibility, yet simplicity. But I'm smart enough to know that it is the granularity/ simplicity of Savage Worlds that really throws some people off, so I'm not a raving, lunatic fanboy about it. It's followed closely by GURPS 4e.
I'm also quite partial to Runequest 2e deluxe boxed edition, but I haven't played it in many years.
My one-shot systems are Dr Who: Adventures in Time and Space and Paranoia.