I used to be a complete gaming nut, so thanks for the excuse to go on a trip down memory lane. Winner at the bottom.
Civilization 3 (20 years old and I play this now. Has flaws but basic modding can fix it to make a more balanced difficulty. I can't play Civ4+ because this, to me, is peak Civ. Playing multiplayer with a friend made lockdown a joy and cost £3!)
Neverwinter Nights (definitely the most time spent on a single game. I'm not really a fantasy buff but I loved multiplayer where you fully roleplay a character. Its like playing out a book you've written in your own head and makes it difficult to go back to single player RPGs where you play someone else's character saying someone else's words)
Syndicate (playing as a terrorist henchman was kind of unique if morally dubious)
Zelda 3 (better than 3d ones)
SFII (biggest single advancement of graphics in the history of gaming?
Secret of Mana (multiplayer SNES RPG)
FFVII (I've not replayed it in over a decade but I think it'll hold up pretty well)
Alien Resurrection (pant wettingly tense PS1 game that still spooked me in the PS3 era)
Megaman 2 (my first game when I felt like I was playing an arcade game. Iconic music and opening credits)
Elite (really showing my age, ludicrously advanced for 1984 and supremely immersive. I played it for the first time years later and it was still ahead of anything else)
Mario Kart (SNES original, best multiplayer to this day?)
ICO/Shadow of the Colossus (I just loved the setting and presentation)
Exile: Escape from the Pit (really obscure and cheap oldschool turnbased RPG which I think you can get for free https://www.spiderwebsoftware.com/exile/winexile.html
. Fantastic sense of exploration and great setting)
Persona 3 (probably the best of the Atlus RPGs although they had started getting too easy by this point)
Deus Ex (PS2)
Ring of Red (PS2 turn based strategy game set in alternate post-WWII setting - only turn based RPG I've enjoyed)
Xenoblade Chronicles (peak action RPG?)
Rez (PS1 shootemup with just an immense final level visually and sonically)
Half Life 2
Portal (for similar reasons to my winner)
Panzer Front (great little PS1 tank simulator with excellent attention to detail and ability to play wildly differing US, German and Soviet tanks in great varied settings. Last level has you having to escape a destroyed Berlin before the Soviets encircle you. Intense scenarios like that and I've no interest in simulations)
Phantasy Star (Megadrive?) - played this on a PS2 megadrive collection decades after its release and was completely absorbed.
Hotel (simple board game with modified rules for my family to give it balance)
Super Cluedo (awesome board game where you get the pleasure of being able to kick opposing players to some random useless corner of the board, really competitive and accessible)
Frequency (PS2 rythym action). Perfect difficulty (for me) and makes you feel like a god as you do things that a week a go you genuinely thought was completely impossible. Numero uno because you can't get that sort of feeling from a book or film or frankly from anything legal.
PS - (I've still got a huge backlog of Wii and PS2 games that ended my game purchasing days, but I cant see anything topping the above. Once that lt is cleared I'll be diving into 1980s and 1990s Japanese RPGs as I find these hypnotic and with a satisfyingly harsh difficulty that is lacking in many modern games that are terrified of discouraging you. Now I hear we can access US versions that were never released in Europe. From what I've seen of PS3 era onwards, nothing really grabs me so I'll just live in the past).