One game... ONE game?!?
Well... I guess the one that I spent the longest playing (partly because it had mazes which actually I hate in adventure games, and partly because this was in the days before the internet and online hints / walkthroughs), and that therefore sticks in my head very prominently, is the wonderful (apart from the mazes) Zak McKraken and the Alien Mindbenders.
At least the mazes weren't as horrible as the catacombs in Indiana Jones and the Last Crusade, which were so bad that the original game packaging included a map, and it was still confusing!!