* Total Annihilation - Just seeing the sheer number of units (thousands) and the sheer destruction.
* Homeworld - Such an amazing and engrossing story, you really did feel you were in the middle of an entire universe.
* Halo: Combat Evolved - Stepping out of the life pod and seeing the open world for the first time.
* Perfect Dark. - Being amazed with lens flares.
* Donkey Kong Country - Pre-rendered sprite work was jaw dropping, who knew the SNES was capable of such imagry?
* Crysis. - The Graphics king for years to come, still holds up even today.

• Spear of Destiny: Back then, I didn't play Wolfenstein 3D. I played its sequel instead, and it's probably the most memorable experience of them all in my first years of gaming. It's not exactly a huge wow but I'll include it anyway as 'the' game of my first years of gaming.
• Metal Gear Solid: The overall experience just was something else. Just wow.
• Metal Gear Solid 2: Final Fantasy X is another strong contender for a game that I feel like must have affected me somehow deeply, but MGS2 definitely takes the wow factor. Unlike in the case of the first MGS, MGS2's wow factor is largely because of its story. It's been a wow more than once too. There's a lot else to wow at as well though.
• Assassin's Creed: I'm really torn between the first Assassin's Creed and Oblivion, but I think I'll have to go with the first AC. It's mainly because of the story and how it got me to sort of think. There's also the 'cheap' plot twist at the end, and it didn't feel out of place, so there's that too, but mainly it's the overall story. I feel like the series changed after the first game, and not all for the better. Oblivion's wow factor and appeal ought to be obvious, but in the end, much of the wow factor had turned into simple appeal after the beginning. By Skyrim, there was barely any wow factor left anymore, it was turned to appeal already.
• Assassin's Creed: Origins: I haven't really had any huge wows yet this generation and I'm not expecting any either, but AC: Origins was a fairly memorable experience that managed to surprise me after all the previous AC games. Metal Gear Solid V is kind of the main contender here, but the main thing it has going on for it is the ending - otherwise the game kind of falls short, especially because it feels rushed/unfinished (probably for a good reason too).

Just for the record, my biggest wow factor ever probably goes to MGS2.

As for non-generational wows, the online multiplayer wow goes to Wolfenstein: Enemy Territory. It just nailed everything, and while I simply enjoyed it back then without really wowing at it, it's kind of a 'wow' now that I look at it again. No other game's online multiplayer has nailed it for me.

PS4 : GOW, Sekiro, RDR2, Horizon, RE7, U4.

Switch : Super Mario Odyssey.

PS3 and 360 : Uncharted series, TLOU, KZ2, MGS4, GTAV, RDR, BATMAN: Arkham City, COD4 MW, Dark Souls, Bioshock, Gears fo War 1 and 2, Mass Effect 2.

I remember being really wowed by the AI in Halo Combat Evolved, it was the first time in a game where I felt like I was fighting actual intelligent foes instead of crude, predictable lines of code.

Late to the party, but this is a good thread,so...
Vic 20 (gen 1 or 2?): Cosmic Cruncher, just happened to be the first game I saw, TV playing games blew my mind. Golf was my favourite.
Commodore 64/gen 2: Moon Patrol - driving around in a crazy armed space tank and blowing up ufos was cool, the side scrolling was neat.
NES / gen 3: Super Mario Bros. Was like the Commodore 64 games I had played on steroids. After trying it out at my cousin’s house I had to have an NES. Honorable mention to Zelda and SMB 3.
Super Nintendo and PC / gen 4: Actraiser - the soundtrack was incredible and the game looked great. It was kind of easy and I eventually traded it in, but the feeling of descending to earth while the music got more and more intense is with me forever. Honorable mentions to Star Fox, Wolfenstein, and CD games with their FMV.
N64, PC and PSX / gen 5: Super Mario 64 - playing this for the first time blew me away, it was the promise of Star Fox fully realized. Honorable mentions to Final Fantasy VII and it’s sweet FMVs, Pilotwings 64, and Unreal Tournament.
GameCube and XBox / gen 6 - uhhh, maybe the sequence in The Wind Waker where Link descends into Hyrule. It was not the most wow gen for me. Honorable mention to Halo
Wii / gen 7 - Wii Sports and Raving Rabbids - played them simultaneously, still enjoy them now. Would like a proper “Wii” Sports sequel on Switch. Honorable mentions to HD twins graphics, no particular game stood out for me.
Wii U / gen 8 - Breath of the Wild: looking out over the edges of the great plateau was mind boggling. Honorable mnetion to Uncharted 4 which looked pretty amazing as my first or second PS4 game.
Switch / gen 9 - nothing specific yet,though like others said, it is cool to play HD games on a portable. Yes, I dare to call this generation 9

I'll start with the easiest.

N64 the jump to 3D was mind blowing, the control stick made it feel to a degree like that time i was gaming on pcs with a mouse.

Goldeneye was mind blowing because of the controls first impact (hell Perfect Dark was a better game lol) and TBH i think the game made the movie. I saw the movie recently and sorta cringed lol

Mario 64 was the biggest upgrade in gaming that I've felt, where I went from playing Donkey Kong Country (which was top-notch SNES) to Mario 64 (which compared to other N64 games doesn't have the best graphics), and I was a kid, so those moments are few and far between.

I don't think anything else has ever had that "wow" impact on me; maybe World of Warcraft. Uncharted comes to mind as an impressive piece for it's time..