I'll go old school on this one.
Impossible Mission on c64, Synthesized speech
"Stay for a while, stay here... forever" Oh god that was great to hear speech on your c64
Any Amiga game by Bitmap Brothers
Those were just so beautiful, I didn't think anything like that could be done
Duke Nukem, Spiral staircase
Even though it wasn't really 3d, but it was done so well in 2d that you actually thought you had the first real 3d engine in your hands. That was wow big time.
Nethack on every little detail
Having silver rings on fingers actually deal extra damage to silver hating creatures when fighting without weapons, but not when wearing gloves.
Joke addition for last
Guitar hero on c64
It actually was done, and it worked, and on 64K RAM, daaaaammmn