SM64
after years on my Grandpa's NES, I was blown away by the 3-Dness. Oh, & TP

GoW, LoZ:OoT, DoW (coming from Warcraft III), Luigi's Mansion lol (I was like omg the torch light!), oh and of course Shenmue on the Dreamcast.

Zelda TP, SMG, RE4, GeoW all made me say wow.

Resident Evil 4, baby.

Also Wind Waker. It still looks fantastic.

Let's see... Chronologically...

King's Quest V: First "hi-res" 256-color VGA game I played. After playing so many 16-color EGA era games, it looked absolutely gorgeous.

The 7th Guest: First FMV-based game I played; the idea of live actors in a video game was incredible back then.

Quake: First full-3D game I ever saw. Though it wasn't much to look at, it was huge since it was actually 3D.

Xenogears: My first exposure to an RPG with a dynamic 3D environment with full rotational mechanics that wasn't limited to the world map.

Final Fantasy X: First PS2 game, and thus first exposure to the higher poly count of that generation of games.

Oblivion: Though the game itself bores me, I cannot deny that its insane visual detail level caught my eye immediately. It was also my first exposure to a decent physics engine in a game, so it gets brownie points for that.

Super Mario Galaxy: Extremely well-done subtleties in environments and events; I've yet to see anything anywhere near as detailed and eye-catching when it comes to background, non-critical details.

Star Fox: Seeing 3D in the SNES was just WOW.

EarthWorm Jim: Probably one of the best looking pure 2D games back then. Animation was extremely fluid.

Super Mario 64: Mario in 3D... nuff said.

Ocarina of Time: Awesome graphics. Everything looked so full of life and worlds were huge.

Rogue Squadron: Do I need to say more?

Star Fox Adventures: Every visual on this game was amazing. From fur, grass, water to fire and the huge bosses.

Wind Waker: Link in cell shading... and still, it was full of life.

Half-Life 2: Do I need to say more?

Farcry: The jungle looked so awesome. And everything looked so detailed.

Crysis: Read above, but make it x10.

Super Mario Galaxy: Mario in 3D + in space + loads of mapping effects + 60 frames per second. You get the point.

 I was actually thinking of throwing in both wave race and RE: the remake for the same reasons.

Final Fantasy X for sure, was an insane improvement on the PS1 graphics!

Ratchet and Clank: Tools of Destruction being another!

Uncharted: Drakes Fortune of course :D

(Only listed games that i have played as thats the only way you can really be sure)

LoZ:Windwaker (GC)

Star Wars:Rogue Squadron 2 (GC)

Jet Force Gemini (N64)

REmake (GC)

Super Mario Galaxy (Wii)

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Edit... I am so glad at the Windwaker love here, Twilight Princess was not a "wow" game to me... although graphically alone the twilight areas were beautiful. 

Super Mario Bros
Super Metroid
Metroid Prime
Twilight Princess
Gabriel Knight Beast Within