I was really impressed by games usually at the beginning of a generation, when we saw new ideas we never thought possible, or towards the end of a generation when the developers are making the hardware do stuff you'd never think it was possible to do.
Sega's arcade games: This shit shouldn't have been possible in the 80's!
Super NES launch: F-Zero, Castlevania 4, and Super Mario World were mind blowing.
Sega CD: Not the crappy FMV games, but the excellent side scrollers with awesome redbook audio, like Sonic CD, Final Fight CD, and the Terminator
32x: Doom! I didn't have a PC at the time, and this blew my mind. Also, Virtua Racing Deluxe.
Saturn: My bud and I rented a Saturn when it was first released with Daytona USA, Panzer Dragoon and Virtua Fighter and played it on my dad's 50 inch TV. Mind. Blown. To this day, this is probably the most impact new hardware has had on me.
PC: Installing a first generation 3DFX card and playing Jedi Knight, Turok, and Wing Commander Prophecy. The PC was so far ahead of the PSX/Saturn at the time it was ridiculous. If it wasn't for Saturn imports, I would have stopped console gaming altogether then.
Dreamcast: I got a Japanese import unit a while before the US launch because I wanted to play Virtua Fighter 3. I wasn't so impressed with the DC port since I had played the arcade game, but the next week Sonic Adventure came out, and it was a religious experience. I knew the Dreamcast would be something special.
There wasn't much after that that impressed me too much, but the SNES, Saturn, and DC are hard acts to follow.

