H.E.R.O.
Atari 2600
Vintage gaming community.
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H.E.R.O.
Atari 2600
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.
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.
Legacy of the Wizard - NES aka Dragon Slayer IV Drasle Family - MSX
Galaga
Zork
Freelancer, PC
Thief (1998)
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!
Dance Dance Revolution.
(Do you feel old now?)
Suikoden
Sonic the Hedgehog 2 - Mega Drive
My first game ever.. good times
Bubble Bobble, Arcade, 1986
Great, now I'll have the music from this game stuck in my head all day
Metal Great Solid
Final Fantasy Tactics
X-COM: UFO Defense
Phantasy Star IV
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.
Warcraft 2 best Warcraft hands down
Street fighter 2 - hyper fighting edition (arcade).
Played this every day after school for years as did a huge portion of my school.