Skyrim

Final Fantasy XI. It's been online for over 20 years and still has a devoted player base. The game's scale is so epic that many people still haven't beaten the expansions.

When it came out it was so far beyond what we had seen in an MMO before - The only competitors were UO and Everquest, but the graphics, music, complexity, and storyline were miles beyond those games.

It's a game with unimaginable depth of play that takes years to master - not like the hand-holdy easy games we get nowadays. Truly a gem the likes of which we will never see again.

Puzzle Pirates, frankly. Made by people who knew what they were doing, were extremely talented, independent, although eventually tried to hook onto Sega as publisher, almost killed the game and then re-purchased the game from Sega to continue as "re-indie" devs. Still going to this day with a stable player base of a few hundred. The game itself is very clearly hand-crafted and every one of the (few) developers left their mark on it. Feels completed and polished.