The original Monkey Island alone beats Sam and Max for me. Maybe it's because I played it first (MI), but there's just more to the game that wins it for me. I love the game dearly, but there's nothing in Sam and Max that tops stuff like the Pirate Leaders or the whole dueling scheme.
I played through the original Sam and Max about a month ago and a lot of the side characters are really short lived, shorter than I remember. None of them are nearly as enduring as all the sidekicks you pick up along the way in Monkey Island and Conroy Bumpus simply doesn't hold a candle to LeChuck. The choice is pretty easy.