The first game that made you realise you suck.

[hsrob]

Gnac said:
F-ZERO GX Story Mode. It seperates the men from the Gods.

What? You thought it was hard? ;)

For me it might have been Empire Strikes Back on the SNES, Jedi mode, i beat it in the end but by god it was hard.  I doubt i'd have the skills for a game like that these days.