for me its the N64....
everything looks blurrier than it did back in the day along with bad early 3D polygons. I don't think I will be sad if i never get to play it again. Basically, I could live without the N64 Zelda's as hard at that might be for some people.
The PSone is a close second except I still enjoy games like SOTN or other 2d games with no 3D at all.