Star Wars: Saga Edition (corebook only) is probably my favorite RPG. Let me start by saying that my personality demands that I analyze things to death by way of tearing them apart and putting them back together. Since beginning to tabletop RPG in the fall of '07 I have played D&D 3.5 (but really, I had no time to learn the system, the guys I was playing with had been playing for a decade), D&D 4th, Legend of the Five Rings 3rd, Star Wars: SE, WEG: Star Wars, Hero 4th, Psion: Hero, Exalted 2nd, oWoD, nWoD, MC:WoD, and the classic Battletech RPG.
I've been to hell and high water figuring out the advantages and disadvantages of simple dice systems vs. complex dice systems, loose rules vs. strict rules, storytelling intense systems vs. combat intense systems, etc. I just might be obsessed. I've had the handicap of playing with a lot of power gamers and/or with a really awesome storytelling GM that always has a personal NPC (his PC) that's better than everyone else's PC at just about everything. I've tried to rise above the power gaming and taken up the mantle of StT/GM on a few occasions and practice is starting to make 'not bad' if I do say so myself.
Back on topic, SW:SE edition really claims my top spot because it's got the privilege of really being D&D 3.5 in space, finely honed by a decade of play testing. It's simple to teach/learn, has specific rules that are almost never ambiguous, is rather well balanced (with a few exceptions), and has a very satisfying pace.
Taking a close second for me is L5R (Legend of the Five Rings) with its incredibly right skewed bell curve 'Roll and Keep' system. The new 4th edition helps to remove a lot of balance issues that were horrible detractors from the ability to enjoy 3rd, but loses none of the flavor or the incredible flexibility. I'll admit that it's not for everyone. Your group has to want to play in the incredibly rigid, romanticized Samurai setting that the game occurs in, but the book covers everything you'll really need to know somewhere in its pages and has great mechanics to go with it.