This is SO hard for me to answer. I used to say "Atlantis" but as I've been considering having to sell my entire collection lately (I lost my workshop and just don't have a good alternative on the horizon) I find that every game in my collection is one I love:
Atlantis
Quick Draw
Melody
Bank-A-Ball
King Pin
Lucky Hand
And even the Big Hit and Jacks Open I'm finishing up to sell will be hard to part with.