RetroGaming

Marathon.

Street Fighter III: 3rd Strike - Arcade

A timeless masterpiece of 2D fighting with beautiful sprite work and a banger soundtrack. Other fighting games come and go, but we still play this one online and in tournaments 25 years later.

https://www.youtube.com/watch?v=YmxOO-A3i7w

Galaga

Bubble Bobble, Arcade, 1986

Oddworld: Abe's Oddysee (PS1)

So many games that I even upvoted that are still absolutely amazing haven't really aged perfectly control- or gameplay-wise, but Oddworld could be released today and it would still be perfect.

Freelancer, PC

Pokémon Crystal is just shy of your 25 year threshold depending on what month it was released (it came out in 2000 in Japan), so I'll say Pokémon Yellow or Pokémon Gold and Silver. We can sit here and nitpick about which is the best, but Red, Blue, Yellow, Gold, Silver, and Crystal were all amazing.

I am very sad that I've scrolled several pages and haven't seen the obvious one (though I could just be blind):

Deus Ex.

Edit: or perhaps it's because Deus Ex is from 2000 and I don't know if it counts yet. Maybe I'll wait 'til next year!

Perfect Dark, N64

Thief (1998)

Sonic the Hedgehog 2 - Mega Drive

Metal Great Solid

Spyro (series) Playstation 1998-2000

It just barely makes the cut off since it came out in 99, but Sid Meier's Alpha Centauri is my favorite strategy game of all time. It even managed to get a Linux port and you can get it on GOG. I was excited when Civ: Beyond Earth was announced that it would be a true successor, but sadly it wasn't even close.

Day of the Tentacle - PC

Warcraft 2 best Warcraft hands down

Heroes of Might and Magic III, PC

X-COM: UFO Defense

sonic 3 and knuckles, Sega Genesis

IK+ (International Karate plus), Amiga, 1987

Dance Dance Revolution.

(Do you feel old now?)

Castlevania: Symphony of the Night

Edit: nevermind, this was already posted. I literally did a search on the page first and it didn't match for some reason.

Metal Slug

Spacewar, DOS, 1985

Final Fantasy Tactics