Well other than playing Conquests of the Long Bow for about five minutes when I was really little (and having absolutely no clue what I was doing), I guess it was actually Escape from Monkey Island, which was one of the first games I seriously played and owned. Of course I still enjoy the game, and moreso than most people (prob. due to nostalgia than anything else), but I won't deny that I don't hold it in the same regard today that I used to. It's not a bad game and i play it everytime I play through the series but usually just to continue the story more than anything else.
Technically, you could make an argument that my first adventure was Indiana Jones and the Infernal Machine, which is action/adventure, and actually it led to my eventual purchasing of Escape (and most of the LA catalogue really). I still enjoy parts of it, but it was frustrating at times even when I played, but dangit it let me be Indiana Jones which was really cool at the time. Some of the environments still look quite nice and I always greatly enjoyed driving the jeep around.
Glad to hear you got Alien Incident working!