lots of agreeable replies so far. I had to think about this quite a bit.

Super Mario Bros. (NES) -- I still never beat it. but this is videogames to me.

Sega Rally (Arcade/Saturn) - pretty much every racing game was an "arcade" racer at this point, so when I finally played this one, it was the first time it felt like you were actually driving a real vehicle. the physics were amazing, and you couldn't just power slide, Mario Kart style through entire races. but it was still arcade-y enough to be fun and not painfully challenging.

Harvest Moon (SNES) - the first game for me where you could do nothing. there was no time clock (aside from the passing of days) or farm monster coming or any real crisis to attend to. it was pure side-quest fun, as a main game. I loved it.

Ape Escape (PS1) - I couldn't tell you shit about this game, I think I only ever played it on a PlayStation magazine demo disc, but it still broke my mind because you had to use both analog controllers at the same time. I was an analog hold out (didn't like the N64 controller) and most 32-bit games were still clunky enough that it didn't matter (remember when 30 FPS was amazing and 15-20 FPS was acceptable?). this game made me realize there is life after the d-pad.

Myth: The Fallen Lords (PC/Mac) -- I was so hyped for this because there wasn't much interesting gaming happening on the old Apple computers. Bungie was only marquee gamedev for Macs (Marathon series) and this was going to be a huge cross-play title for them. the reason it makes the list, however, was this was the first game where I enjoyed online gaming. it wasn't an easy game to play, but you didn't just get bulldozed like FPS games (Doom, Quake) if your skill level wasn't elite. so many great memories, and the first and only time I was ever part of an online gaming community.

NFL Gameday '98 (PS1) - if you were a gamer during the 8-bit, 16-bit and 32-bit ages, you definitely have a ton of crystalized moments where you felt the technology taking you to new places. watching fully polygonal football players, with smooth motion capture, move around the field was one of those moments. it made Madden look like pure dogshit and sports games were never the same.

Metal Gear Solid (PS1) - this was the first defining "game as a movie" for me. the playable beginning, where you get shot from a torpedo tube and sneak into the elevator, then the opening title rolls as you ascend…just the first of many amazing sequences that had never been experienced before. the next generation of gaming went too far with the cinematic concept (and we found out most game directors would be shitty movie directors), but this experience will forever be an unassailable landmark.

Half-Life (PC) -- this came out right after MGS, but it took me a while to play it since I didn't have a gaming PC at the time. I know there are predecessors in the FPS genre that did a lot of the real-time story-building that Valve accomplished, but like MGS, this was the complete package for me and changed how I thought of FPS games. I still have a mental scar from the first time I got pulled towards the ceiling by a barnacle.

Street Fighter IV (PC) - I played Street Fighter since II in the arcades, Super Street Fighter II: Turbo took away so many hours when it hit SNES, I would say that Alpha 3 is one of my favorite games of all-time, and I remember watching EVO Moment #37 not too long after it happened. that said. the coalescence of IV reviving the fighting game genre and later, live streaming through Twitch, made me realize I've been playing fighting games for 20 years but had no idea really how to play them. plus/minus, frame traps, mix-ups…the FGC became a whole new world to me and occupied most of my gaming hours for the next several years.

Final Fantasy VI (SNES) - I can't really pinpoint which was the first great RPG I played, since I got into them all at once -- from 1993-1995 I remember renting Secret of Mana and Chrono Trigger from Blockbuster, while scanning FuncoLand ads for used Final Fantasy II/III (US) cartridges that were less than $70.

but I do know that Final Fantasy VI was the RPG that made me a fan for life. the imagination, the art design, the ensemble cast that all had their own characterization, the gotdamn best villain ever, the 16-bit soundtrack that blows away most orchestrated attempts to rival it, the beautiful layers to the story -- fuck, how many games since then have had moments that come close to Setzer piloting Daryl's airship out of her tomb? which happens right after Celes on Solitary Island?

videogames had always been fun, but this is the first one that made me "feel" something. a perfect gaming experience.