The first couple episodes from Sam and Max Save The World were good, I thought. Haven't gotten around to the rest of them yet though, so dunno if the humor continues to hold up.
If you're a Wallace and Gromit fan, you'll probably like Grand Adventures. (Not the same VA for Wallace as the show, but he pulls it off well enough.) If not a fan or unfamiliar with it... might be better to give it a pass, or at least check out an episode to see if it's your kind of humor first.
Haven't played Back to the Future or any Monkey Islands yet, so can't personally say anything on those. Heard good things about both though, for what that's worth.