Well, let's see. Over the last five years, I have played in the following:
Shadowrun, 4th Edition
D&D 3.5, variety of settings, including several homebrew settings, plus Monte Cook's Ptolus
oWoD Vampire (got roped into it by gaming group in the early days; it was that or nothing)
nWoD: Changeling
Call Of Cthulhu
Paranoia
And have GM'd the following:
Shadowrun, 4th Ed.
D&D 3.5
nWoD: Mage and Geist (combined setting, modern London)
The Dresden Files RPG (FATE System married to the Dresden Files setting)
Diaspora (Hard scifi FATE)
Tethys: The Ten Thousand Coasts (personal homebrew maritime high fantasy setting done with FATE)
And have extensively looked at, made notes for, or desire to play:
Eclipse Phase
CthulhuTech
Deadlands
Mutants And Masterminds
Gamma World
Star Wars
Spirit Of The Century
D&D 4e (mostly as an intro=level game for some of the students I tutor)
Now, out of
all of that, do you know why SR4 is one of my favorites, if not my absolute favorite setting and setting, that I was not only willing to return to GMing it after five years and a really bad experience as a player?
Several factors: first, I have a personal philosophical issue with Level Advancement Systems, especially Level And Class advancement systems, most especially the way that they're often played. To whit, *DING!*
The idea that you hit some threshold of experience and everything about you improves. So, while I'll play D&D, and enjoy it, there will always be the edge of discomfort with the "Woot! Hit a new level!" conceit of the system. Thus, I'll always be more comfortable with Point Expenditure systems of advancement.
FATE is a phenomenal system for storytelling, as it codifies the elements of stories--the Aspects that drive the plot are only story elements
because they drive the plot--as well as having a solid mechanic that allows for PC control of said plot, as well as incentivising actual bad things and complications for the PCs. However, while being very easy to play, I find the system lacks nuance: when everything ultimately boils down to a normalized bell curve from -4 to +4, and an aspect can be anything from MYSTERIOUS AURA OF MENACE to a location aspect like DEEP FRYER (in a kitchen, perhaps), you can loose alot of nuance, and, while I adore The Dresden Files and love the setting and story, getting a deeper level 0f nuance is difficult in that system.
WoD has nuance, but the conceit of the setting is an utterly craptacular world, in its own way worse than Shadowrun (depending on the GM, I suppose, but especially oWoD), but there are sufficient options and degree of finesse inherent in the system to allow for a great deal of nuance.
And that leads me to Shadowrun: there are enough options in Shadowrun that creating a unique character is ultimately quite doable, and allows for a great deal of, well, nuance in that creation. And, unlike nWoD, which is a superior product to its predecessor, there is an explicit and engaging
world, already made and fleshed out and ready for you, a unique setting, with a distinctive flavor that is married to that nuanced system.
And
that's why Shadowrun is my favorite, over and above all of these other games that I have tried and, yes, enjoyed: because it offers something that no other game does, that something being an incredible array of choices and options, married to a strong, rich setting that has no peer out there in its depth, history, array of possible play options and scope. I wanna run a horror-genre game? Well, there's CoC, or CthulhuTech... or I could run a bug hive campaign in Shadowrun. I wanna play a guy that's Cursed With Awesome? nWoD is a possibility, but designing a setting is a pain... or, I could run a changeling, or an Awakened with the Cursed quality, or a newly Awakened Adept from a magephobic family... I want deep and horrible conspiracies that will shake my very worldview and cause me to question everything I thought about history and the world? CoC... but actually having a character, y'know,
survive past the first session can be a nice thing, too. And if a dragon running a corporation isn't enough for that particular desire, I'm not thinking hard enough--and there's always the option of master shedim, too. And so forth.
tl,dr version:

Shadowrun offers possibilities and combinations of possibilities that I can't get from other systems, and has them all in one convenient place and ruleset, and that's why it's my favorite.