RomeoReject :
"The test of time."
Mostly games from this decade.
That's what happens when you use incentives to encourage users to nominate things. Most who took part in it did it just to earn XP to level up their profile, and didn't actually care all that much about what they were nominating. So you get a bunch of popular games that in some cases only have tenuous ties the category.
Age of Empires II fits the category fairly well, since the original version came out in 1999, and while TF2 was from less than a decade ago, it's remained quite popular, so it's arguably a reasonable addition as well. The others are a bit of a stretch though, with all of them being only around 5 years old. I guess they could be considered games that you can play for a long time and still come back to, but do they really stand "the test of time"? Within a couple years, most Civ 5 players will have moved on to Civ 6, and most Skyrim players will have moved on to the next big Elder Scrolls game. Terraria might not be all that popular either once the updates end and a suitable successor comes along. I suppose few games from the 90's and early 2000's remain both replayable and popular enough to make such a list though, which ultimately comes down to being a popularity contest.