It's harder to do this with games than with movies. For example, how do you compare Everquest, Overwatch or FIFA with artistic games like Gone Home or Dear Esther? When rating movies, even though you're comparing a short comedy with a 3-hour thriller, you can at least still compare the acting, the cinematography, the cutting, the sound, etc.
Games are much more different from each other, and while some games are meant to be enjoyed maybe an hour or two of your life, other games are meant to take up potentially tens of thousands of hours.
MWO is the game I've sunk most hours into of all games, I think. But would I say that it's better than games like Gone Home, Dear Esther, Sword & Sworcery, or other short games with a very specific artistic vision and a lot more attention to detail and craft? I don't know. How does one compare MWO with Day of the Tentacle or Indiana Jones: The Fate of Atlantis?
I think the only reasonable way to do it is to compare different FPS games and then judge them based on how good they were for their time. Not just in terms of how much you played them, but whether or not they pushed the envelope, did something new, or actually had some deeper emotional value as artistic products, or whether they had a compelling story that stuck with you.
http://www.pcgamer.com/best-fps-games/
PC gamer divided FPS into different categories, like best single player, best co-op and best competitive (PVP) games and put MWO on the list of best competitive FPS games ever made. I can agree with that.
Is MWO better than Loom or Commander Keen or Civilization? I have no idea how to compare them. It's like asking whether a certain wine is better than a Big Mac, to me. They're different things.