I know it's very archaic compared to pretty much everything that came after it, but I still love Maniac Mansion. It's more open-ended design and alternate story paths from playing different characters was really ambitious and something rarely attempted by other adventure games. Even Day of Tentacle is nothing like it despite being a sequel. Though I still can't deny that the Indiana Jones adventure games topped Maniac Mansion in how they handle alternate paths. Those are a close second I think.