This is a tough one for me, because it really depends on my mood. Hmm.
[An hour later after writing out a list of favorites and arbitrarily narrowing it down in a Sweet Sixteen style bracket fashion...]
Okay. After giving it way too much thought and agonizing over the answer (since sweepstakes would know if I lied, of course), my pick for "all time favorite game" is....
"Sam & Max Hit The Road."
This was a tough choice! SO many good games. I almost picked Knights Of The Old Republic, or the first Fallout, or Curse of Monkey Island*, but the first Sam & Max game was practically life-changing for me when it came out 27 years ago, and so many of the in-jokes with my wife stem from random lines from this game. I can't count the number of times we've had reason to say "Are you as confused as I am?" - "Moreso!"
So, while I'm by no means saying it is "the best game ever," at the end of the proverbial (or is it metaphorical) day, Sam & Max is my favorite. It's the only game for which I've bought merch and pursued every iteration of the characters, even their "meh" animated series. And I was thrilled when they showed up in TellTale's cameo-riddled poker titles. I'm not a big poker fan, but I play those just to hear Max's lines.
(* - Honorable mentions also go to Skyrim, Civ V, Age of Empires III, and Gabriel Knight: The Beast Within.)