Sword of Spirit
Legend
Second place is probably Fiasco. Which depending on definitions might not qualify as an RPG. Or it might. Tabletop gave it the chance to break out.
I would be interesting in seeing if we can get RPGs to a larger audience by producing more "gateway RPGs" like that, and getting them into mainstream party gaming.
Don't tell them they've just played a "role-playing game" until after they are hooked...
But there's a huge issue here - it's very hard to make an industry out of rules light RPGs. Supplements are much harder to write and people who want them don't think they need more. So the industry is always going to lean to heavier rules than the players want.
Good observation. High quality settings might be the key. GURPS setting supplements apparently used to sell really well outside of GURPS itself, since they were known to be of such high quality and were mostly fluff. If game products made for an entirely different system can sell for their fluff, basing a mostly fluff book for a rules-light system, and making it very high quality, should be able to sell well to those who like the system (and maybe spread beyond it).