Well...
World of Warcraft. It's debatable whether or not it's worth how much I've ended up spending on it, with a recurring subscription for about five years now, but across the three characters I usually play I've pulled up something like 280 days of played time in that, so I'd say it's been worth it.
Skyrim. As someone who couldn't get into either Morrowind or Oblivion, Skyrim seemed to have something different that just let me dive right in and love it. Maybe it was the vastly improved third-person camera, or the gritty-yet-beautiful game world, I don't know, but I put in well over 80 hours into that.
Kingdoms of Amalur: Reckoning. There isn't enough to say about that game. It may not have the best story (but lets be honest, neither did Skyrim), it may not have the best graphical fidelity or engine, and it may be styled very much so like a single-player MMO, but I love it all the same. Great combat, great aesthetics, great sound design, and an absolutely massive world.
Metal Gear Solid. The series as a whole, really. Sure, they're 3/4 cut-scene and 1/4 game-play, but at least Hideo Kojima knows how to make the cut-scenes interesting. And lets you skip them (Though I wish you could pause them). And damn does he know how to write some characters. Disregarding the lack of an editor with the balls to cut down the scripts, at least most of the characters in that series are very well defined. Even Raiden had a great character arc in MGS2.
Mass Effect. Again, the series as a whole. But if I were to pick an individual game out of the series, it'd be Mass Effect 2. Got it on PC and PS3, put in about 60 hours on the PS3 and about 50-60 hours across two characters so far on the PC.