Personally, the systems I prefer are SR4, D&D 3.5, M&M2, and Hero 5th. With the exception of Hero System, they are all fairly simple systems, that can scale easily to different power levels. Hero system, of course, is anything but simple, but it provides a very good tactical experience.
Personally, you would have to pay me (very well) to even touch mechanics like SR3, Heroes Unlimited, Marvel's FASERIP, D&D4, or M&M3. Especially the last two. The creators of those two drekheaps need to be thrown on a pyre of their own sourcebooks.