Sid Meiers Alpha Centauri. Arguably the best Civilization type game ever and a great 4X game. There's been no direct sequel and the spiritual successors don't hold up.
Yeah this is a truly shocking one to me.
SMAC remains unbeaten essentially for two reasons:
1) Gameplay - It simply plays better and is designed better than almost all CIV-type games. It has a higher learning curve than most CIV games, but once you get past that, you find that, not only are there a bunch of genuinely very diverse factions, who have stuff about that matters deep into the game, rather than just at the start or specific periods of the game, but a much wider variety of strategies, and more nuanced strategies work than in other CIV-type games.
2) Setting - SMAC was relatively original, certainly inspired by a lot of writers, but with a real grasp on sci-fi, and an eye for less trope-y, less obvious, more thoughtful, and more philosophical SF. There's bits of Kim Stanley Robinson, of Frank Herbert, of Stanislaw Lem, of Vernor Vinge, of Greg Bear and all sorts of others. And rather than being sort of cheaply centrist (as the mainline CIV games increasingly became), it allows for any kind of different or bizarre society to potentially flourish, from the hellish to the utopian to the simply strange. The whole thing is extremely imaginative, and strong and consistent visual design (which could be initially off-putting, but knew how it wanted to look) really helped.
A number of games have attempted it, and they've all failed miserably - probably the absolute worst being Firaxis own CIV: Beyond Earth, which was very much marketed by by Firaxis as a "spiritual successor" to SMAC, but had a number of key flaws, primary among them being:
A) Neither of the lead designers had played SMAC significantly (one hadn't played it at all). This was beyond astonishing, but came out in a couple of interviews. I believe much later, after the game wasn't very successful, they tried to retcon this and say actually they had, but given that they'd rather proudly denied it earlier, and even said it "wasn't necessary" to play it (i.e. implying they had zero to learn from it), that just seemed weird.
B) Said lead designers clearly had poor imaginations, and when they listed their inspirations, it was basically "Super-mainstream sci-fi movies from the 1980s and 1990s" (and most it not space or planetary SF, rather stuff like Terminator!) and a few more mainstream and less imaginative/visionary SF novels, but IIRC, not a single one of them from after 1980, so excluding people like KSR, Vinge, Bear, and so on.
C) Gameplay-wise, instead of being an original game with truly unique mechanics, it was just CIV mechanics transported to another planet, essentially.
Other older games I sometimes still play:
Streets of Rage 2 - I've actually stopped with this, because SoR4 is a "true" SoR game and basically has everything I liked about SoR2 but better (you can even play Shiva, as we always dreamed of back in the 1990s).
Dragon's Dogma - 2012 so I think 11+ years counts as it hasn't been updated, unlike say Skyrim or Mass Effect. A truly amazing game no-one has really equalled in a variety of ways.
Super Mario Bros - the original SNES version.
F-Zero - Original SNES again. Nothing quite like it.
WipEout - The first one and 2097 only - the later ones maybe technically have better gameplay, but none of them have remotely as good and consistent visual design, nor as banger soundtracks (indeed some of the later soundtracks contain some actual sound-crimes). Together with F-Zero it represents an essentially dead genre (modern attempts have been fairly pathetic).
Crazy Taxi/Crazy Taxi 2 - Amazing. Pity they ruined the music because it might have cost slightly more to not ruin it. I should find a mod/hack to put the real music back.
Doom/Doom 2 - Still great.
Mid 90s games felt archaic and outdated even 10 years later, but late 90s games still hold up as things you can recommend to people who only got into videogames over two decades later.
A lot of late 1980s and early 1990s games hold up about as well as late 1990s ones. It's more like there's a whole bunch of games from about 1995 to 2005 that were utterly forgettable for various reasons (even more so than earlier or later periods).