Ok, I'll bite. I love lists too, so... here's one from a guy who grew up with a C64, graduated onto Master System, then Megadrive, then stayed with PC gaming up until the present day. And now makes them, or at least tries to!
Now I'm going to totally cheat and throw out some "alternative choices" for each spot that I've chosen. So with that in mind, here's my top five (with an alternative top five that might well have been my actual top five on a different day):
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5) "Half-Life 2" (PC). If the "System Shock" games fully immersed me in a perfectly self-contained world, "Half-Life 2" made me feel completely part of a larger, more vast experience - which is fairly ironic considering that the "Shock" games were open-world, and "Half-Life 2" is not. Nonetheless, "Half-Life 2" managed the rare feat of having a linear storyline and yet constantly making me feel as though my actions were driving the plot, as though I was truly the hero. It has fantastic FPS gameplay and some great characters (my favorite being the mad preacher of Ravenholm).
My alternative choice for fifth place would be "Bastion", another game of a very different genre but that manages to succeed for exactly the same reasons that "Half-Life 2" did - the gameplay is fantastic and the story, even though it's fixed, is totally immersive. I always felt that I was driving the action in "Bastion".
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4) "Eternal Champions CD" (Mega-CD). Yeah, this is going to be the "out-there" choice. But looking back... this game either predicted or perfected so much of what's popular in fighting games today. The massive lineup of fighters (I think it was, at the time, the largest lineup by far); several secret characters; multiple special moves, death moves, even "cine-kills" that used full-motion video for the kill animations... all grounded in an immensely playable and well-balanced game. The Megadrive version of this game was sadly inferior in every way, with unbalanced characters, a decent but kinda pointless "training mode", and mechanics that just didn't feel as fluid as the CD version. Plus only the CD version had finishing moves that involved your opponent (or you, if you were unlucky) being stepped on by Godzilla, eaten by lions, or crushed by a car full of evil clowns. (Yeah, that one was terrifying even if you WEREN'T the guy being killed.) This was a combination of "Street Fighter" and "Mortal Kombat", taking the best elements from each and improving on them. It's a crying shame how little recognition it gets today.
Alternative choice: "Street Fighter 2". Look, do I even have to explain this one? Everybody over the age of twenty knows this game, everyone's played it, everyone loved it. I had so much fun with my friends trying to sucker them into a dragon punch or spinning piledriver. I grew up with this game.
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3) "Fallout New Vegas" (PC). If you could combine Fallout 3's world with Fallout New Vegas' main questline, characters and core mechanics, you would quite possibly have the perfect open-world game. Separately they're just fantastic. I love the "Fallout" world - the music, the lore, the humour, the fact that the game fully immersed me in the struggles of the present while always keeping a regretful eye on the past. Don't get me wrong, "The Elder Scrolls" series is a personal favorite, but its characters are one-note and its various factions and mechanics barely seem to co-exist in the same universe. "The Elder Scrolls" games never quite gave me the feeling of being part of a single unified "world" in the same way that "Fallout" did.
My alternative for this spot would, of course, be "Fallout 3". The main quest, the skills balancing, and the random encounters system are all huge negatives. But even they can't take away from the impact this game's world and characters made on me.
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2) "Jungle Strike" (Sega Megadrive). The gameplay of this - except for that damn stealth fighter section - was just perfect. "Jungle Strike" was one of the first truly open-world games that I played; and while it doesn't have the interactivity or the randomness of a "System Shock", it doesn't really need it. This game took what the Megadrive could do and pushed it to its limits, delivering a game that I played through many times over. It's also one of the first games that taught me how compulsive it could be to really WORK at a game, to try and find different ways of doing things, and to look for every secret. Of course the reason it taught me that was because it was just so much damn fun.
Honorable Mention for this spot would be "Warcraft 2" (PC). Another compulsively addictive game where I just had to try and find the best ways to complete each mission. It was also one of the first games I played with a "level editor" and I had enormous fun with this.
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1) "System Shock" (PC).
"System Shock" was the game that taught me that videogames could be an experience, not just a challenge to be overcome. To this day the world of Citadel Station has immersed me more completely in its own story - which became MY story, because this more than any other was a game in which you made your own story and have experiences that are unique to you. This is the nearest thing to a perfect game for its time that I've ever played, which is why I'd put it at #1.
Honorable mention for first place "System Shock 2".
"System Shock 2" was just as immersive, and to this day I've never heard a game with better sound design. I've completed it at least twenty times, and will probably keep coming back to it when I'm seventy years old. It still has the power to suck me in every single time. As much as I really like the original Bioshock (especially the first two-thirds), the two "System Shock" games were Ken Levine's masterpieces. I'm hoping he outdoes himself some day, but I don't see how he can, given how extraordinarily high he set the bar with these two games.