If you think a game should be higher or lower in ranking, then upvote or downvote it. If you don’t see a game on the list that you think should be on the list then create a top level reply with the name and system.
Galaga
Zork
Perfect Dark, N64
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!
Freelancer, PC
Thief (1998)
Dance Dance Revolution.
(Do you feel old now?)
Suikoden
Sonic the Hedgehog 2 - Mega Drive
My first game ever… good times
X-COM: UFO Defense
Metal Great Solid
Final Fantasy Tactics
Bubble Bobble, Arcade, 1986
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.