Did you ever buy a console from, say, a garage sale, or off of the Internet, that in its day you never would've bought? If so, what games changed your mind?
For me, with the Atari 5200 I saw at a townwide garage sale back in 1998- in spite of the broken ports (especially Port One, which makes a 5200 unplayable)- it was:
Berzerk
Defender
QIX! Especially Qix!
It was worth the hassle of getting the thing functioning, although it wasn't until recently that I finally got it running completely right.
In mid-2000, there was an NES in a garage sale (townwide again). I bought it mainly for a four-screen DK, which I did play a couple of years before. What made up my mind ultimately, though, were:
Galaga
Elevator Action
Ghosts `N Goblins
Terra Cresta
Donkey Kong 3!
All of which were in the Dutchess Community College arcade at that point, except for DK3 which was in a nearby pizzaria.
I did not know until recently that Super Sprint was available for it, too.