With a few exception period already mentionned D&D has always been number one. The reason for me is simple. D&D is generic enough so that any style can be reproduced with the rules. This is not so with many of the other systems out there.
Although a fan of the work of Lovecraft, I am still surprised at the success of CoC as a RPG. Sci-fi and horror are particularly niche in genre (as ttrpg goes) and from experience, niche genre excitement fades over time. YMMV on that though... But it is also why I do not do every campaigns in Greyhawk, FR, Dragonlance or Ebberon. We change the setting from time to time to shake things up. This both renew the interest and shake up the table with new (or old ones we are fond of) ideas.
I really think that the generic aspect of D&D is its greatest strength. After all, Cthulhu is Cthulhu and LotR is LotR just as Star Wars is Star Wars. At some point, their system is so integrated in the setting that to change setting means changing game system altogether. Not so with D&D. So many settings in D&D gives a unique chance to mix and match so much.