Well it's subjective, as you can't really define "best" game. Also a distinction between console and PC should be made, but my personal favorites (also highly rated on IGN PC) are:

Super Mario Bros (NES)

Tom Clancy's Rainbow Six , and Rainbow Six 3 - Raven Shield (For PC)

Neverwinter Nights and its expansions (for PC) (Original Series by Bioware *Not part II which was ruined by Obsidian)

Age of Empires II (for PC)

Medieval II (For PC) (Possibly the funnest game I've ever played)

Agree also with DJSyle with Need for Speed franchise as that can get pretty addicting.