For me it's Red Dead Redemption. During the last console generation I owned a Wii and a PC and that served me well. There wasn't a single game in that period which I really really wanted to play but couldn't on my Wii or PC . . . except for RDR.
I read about that game when it first came out and LOVED the idea of a deep in depth open world game set in the Old West. Just reading about it got me excited enough to watch a couple of old Westerns that I never would have other wise, but I didn't have a PS3 or XBOX 360 so I figured I'd just wait for it to come to PC.
It's been a hell of a wait. SIX years to be exact. Since before the birth of my oldest child who is now begining to read! And every year, like literally every single year, I have seriously came close to buying either a PS3 or an Xbox, JUST to play that one game.
But . . . wants are infinite and resources are finite. I just haven't been able to justify buying an entire system in order to play one, and only one, game so I've yet to play Red Dead Redemption.
At this point maybe I've built it up too much in my head but who knows? I don't! Because, again, I've been seriously wanting to play that game for six fricken years now.
But what about you? What is that one game, that ONE game that has been sitting in the back of your mind that you have always wanted to play but for whatever reason have yet to do so?