Well, according to Gears of War designer Cliff Bleszinski, it is...
Sci-fi role-playing franchise Mass Effect. Here's the quote — from an interview with website Gamasutra:
The thing that was cool was that Mass Effect is kind of an RPG for me, with the cover-based shooting. I think there's a reason why it's been successful. Because gamers love Gears.
It's like well, here's something that's like Gears, but even has more depth, and a different setting. It's Star Wars for this new generation, right? Well, and the addition of the potential for having sex in a video game is also very powerful.
That could be a throw away quote, sure, but I'd say that the Star Wars for "this new generation" is, wait for it, Star Wars. Nothing against Mass Effect, nothing against Dude Huge. Star Wars, as much as George Lucas continues to mangle it, still has wide appeal with younger generations.
More in the link below.