We're playing two games a week, both set in Ptolus using (mostly) the same players.
One of them is free-form anything goes style 3.5e D&D pretty much as Ptolus is meant to be played. The other same is much, much grittier and deals with the evil daily undercurrent of life in Ptolus. That's using Warhammer RPG, and it's a perfect match. As one of my players commented: D&D is the Justice Society. Warhammer is a corrupt beat cop in Hell's Kitchen.
The two games are operating in tandem is an intertwined plotline, and it's working remarkably well.
When we need a break from the OTT cinematic optimism of D&D or the bleakness of Warhammer we crack open
Microlite20 (yay!) for a session and had a blast. My last session with that was set entirely in a masked ball, complete with a CD of 14th century dance tunes running in the background. Oh, and murder most foul. Oh yes.
Beyond that I've played Traveller (a lot), Rolemaster (even more - and still my all-time-favourite system), classic D&D (the Rules Cyclopedia is the best RPG resource ever created. Ever) and more superhero RPGs than you can shake a Galactic Atomizer at. HERO system ended up being the system of choice, but that's only after wading through MSH, Golden Heroes (remember that?), DC Heroes, V&V and more. I guess we liked the mega-tweakery of it.
If I returned to superhero gaming now though (and I will. I've a plot planned), it'll be with Mutants and Masterminds, because I'm itching to try it out.
Overall it's a mixture between rules-lite (M20) and rules super-heavy (Rolemaster). I guess that I look more for consistency in the rules (hence, I hate 2nd edition AD&D with a passion) over quantity. So long as they make sense, they work for me.