I was going to preface this saying it's just opinion and blah blah blah nostalgia tints things and it depends on what you're into, but the truth is there really is an overall best game of all time. It's a game which transcends genres, a game so good that it has yet to be re-released on one of the modern platforms due to the gaming companies knowing it would make all their AAA games obsolete. That game, is
<span class="bold">Wonderboy III: The Dragon's Trap</span>.
There are many other Wonderboy games. There's even [url=http://en.wikipedia.org/wiki/Wonder_Boy_III:_Monster_Lair]another Wonderboy III[/url], but it's not The Dragon's Trap. The Dragon's Trap is an early example of a light metroidvania, in which you travel the globe defeating dragons in order to absorb their form to access more areas, more dragons, more forms. You can kill a robot for example, which obviously lets you turn into a dragon. Being a dragon lets you breathe fire, allowing you to reach a pig dragon or something which turns you into a mouse, letting you stick to walls. Watch Dogs can only wish it had this much depth, 25 years later.
Wonderboy taught me how to love. When gaming reaches a point where games are seriously considered as possible to be art, Wonderboy will resurface and teach people that not only was it always possible, but that the Mona Lisa has already been created.