There is one correct answer to the titular question, but it may not be popular... There is no one game to rule them all... because rules matter, and rules and playstyles need to be non-exclusive of each other. (Noting that there are playstyles where the rules are entirely "Trust the GM to decide fairly" - even tho' that's one I am psychologically unable to play, it is a valid playstyle. And other RPGs that envision nearly to almost exclusively boardgame or minis wargame mode.)
Now, are there games that deserve more play? Hell yeah!
I'm fond of all of these:
FFG (now Edge) Star Wars
FFG (now Edge) L5R Fifth Edition
Twilight 2000 4E
Sentinel Comics
Pendragon (any edition, tho' 4th is my favorite)
Tunnels & Trolls (again, any edition, tho' 5.5 is my favorite.)
2d20 Dune. (Best 2d20 version I've read, and I've run a bunch of it.. And read many of them)
Alien
Vaessen
Talisman (The RPG, not the boardgames, tho' the Talisman Adventures board game rocks, too...)
Monarchies of Mau. (Pugmire, but Cat-Flavored instead of dog-flavored)
For the lighter fare, I'm quite fond of 4:
My Little Pony: Tails of Equestria (A group of adults taking it for humor but playing the characters sincerely can be a total hoot... System is fairly solid, if simple. And the Zombie Pony teaching the lesson of Sharing to the mean diamond dogs was priceless.)
Kobolds Ate My Baby (All Hail King Torg!)
The 9th Level Games version of Ninja Burger. I actually ran a campaign of it once... the varied and bizarre things players will do to make silent deliveries...)
the old GW Judge Dredd. (I wish Rebellion would have GW reissue it...)
Of that long list, the ones not played in the last 4 years are Ninja Burger and Judge Dredd.
The ones not getting campaigns: KAMB, MLP:TOE, T&T, and Monarchies of Mau...
My campaigns tend to be 3-4 months, and I tend to run 2 at a time.
