I've invested too many hours to count into video games, but as far as single sessions are concerned I think Yie Ar Kung-Fu may take the top spot. It was Christmas of maybe 1987 or thereabouts where I put in I believe around 8hrs non-stop, obsessive? unhealthy? Very likely, but also a lot of fun.
I pushed the game to its limits and learnt that it basically has no end, after you defeat every opponent it just starts again, and again, and again, and again.
It's an early one on one beat em' up style game, before we had Street Fighter, Mortal Kombat, Tekken etc we had the likes of Yie Ar Kung Fu and it was glorious.
You'd take turns battling your foes who are varying martial artists and later some weapon wielders before fighting your arch nemesis (Who is basically a clone of you).
The difficulty curve was actually quite low after you master the controls and learn that one particular move was overpowered and started spamming that!
Regardless of this flaw the game had a lot of content, was very addictive and set the stage for all the incredible games that were clearly influenced by it.
A true retro classic.
The Good:
Great characters
Excellent combat
The Bad:
Never ending!
One overpowered move
