Man this is really tough.

I'd say ME but I'm not too certain of It's future even with ME4 being in the works.

I'm going to go with Halo, because 343 looks like they are doing a really good job. I can expect a new game every two years or so which is fine with me. Multiplayer is pretty much everlasting. I put in over 2000+ hours of Halo 2 MP on the original XBox, and while I may not top that with Halo 4 I can assure you I will come close.

Tekken never gets old that or Little Big Planet cause its a platform for games ;)

Elder Scrolls seems like a good choice, hundreds of hours, with mods that can expand the game, along with the mmo coming out.

Additionally, Final Fantasy seems like a franchise that can last forever.

StarCraft. - Been playing the franchise either daily/weekly for almost 15 years, enough said.

It would without a doubt in my mind HAVE to be some type of open world game that has the option of multiplayer. FPS games will get boring after shooting the same person 50k times. Something more open world and a sandbox type game like GTA, Saints Row or Just cause 2 would keep my attention and with the added addition of multiplayer I could do stupid shit with friends. Endless fun.

Mine would be Zelda or Fallout because I have the most fun with those games.

I would go with Might&Magic, if there was actually any chance that there will be (official) M&M X and continuation of franchise - than again, Ubisoft might (as current IP owners) finally realize how big M&M once was and try to go after Elder Scrolls audience in the future. But since it's highly unlikely, I'd probably settle for Fallout (though not a huge fan of Bethesda's vision of what that game should be) or GTA.

Fallout on PC for me. With all the many, many incredible mods out there, I've put hundreds of hours into Fallout 3 and Fallout NV. It's just a nearly endless experience with so many choices to make. I still want to go back and play New Vegas where I kill everyone and leave the Mojave a bloody, lifeless wasteland. Plus the series has some of the best, longest-lasting DLC ever created.

Dragon Age gets honorable mention, I love the setting, story, and style of strategic combat. Had Suikoden continued, though, it would probably have been my Number One. Actually, had Final Fantasy Tactics continued at all, it definitely would have been my Number One.

Monster Hunter

Hmmm...that IS a good choice. I hadn't considered Elder Scrolls, but that's a really good bet. The games are great, they can be played pretty much forever even after the main quest is over, plus like you said the MMO. Really good choice, Botman.