umm... i don't think there's really a set right or wrong answer for that question. Everyone's opinions are different, and not everyone likes the same thing. Someone may hate a game, while others may love it(which is why we have shoot-em-up games, and rpg's and all those other things). There is no 'best' game, there's only 'great' games (and some bad). If you really want to go for other's opinions, you could ask for the best rated games, or maybe just some that other people would recommend.
So there's really no real answer to that question since it's entirely opinion based.
common question gets a common answer: there is no "best" game. period
try narrowing things down by asking about a certain genre or something similar. best game depends on the opinion of who your asking. like, for example Pirate Galaxy, some people might think its great but imho its increasingly unbalanced as you progress and just frustrating in general. its why i rarely revisit the game and when i do i tend to hang around the 1st or 2nd systems helping newbies with stuff.
though i would be interested to know of any good "endless" games similar to the following: Lords of Lords, eternal war, or the lost runes (try googling to find them). that seems like a dying type of game and increasingly rare to find as far as i've seen
I think the two best are War Thunder and World of Tanks. Sorry if these games are thought to be outdated, but a lot of people still play these games, and they're just plain fun. You don't have to pay to play, it's easy to learn (I skipped both tutorials, and I'm doing just fine), and, once again, it's fun. there is a link to sign up for a War Thunder account on my profile. Trust me, it's safe. I used it myself to make sure of that before I let others use it. See you in the skies and on the battlefield. *Salute*
bit of a necro post perhaps but still relevant. http://euotopia.com/ its free to play (has a microtransaction area but thats nothing game breaking by far as it doesn't affect your loot/lvling rates in the slightest) and is kinda like the real old Ultima games, all 2d graphics.