On the subject of "cult" and "underrated" movies, I think it's really hard to have those these days as everyone can access most of them very easily and of course there's the internet having much info/debate etc
Some damn good films on here though not sure if Cult and Underrated apply to them these days however.
Two I would say that were underrated films at least on release and shortly thereafter would be Time Bandits and Withnail and I. I also think to have be a "cult" following I think a movie has to spawn a "cult" audience Withnail certainly does that, so does The Big Lebowski, not sure something like Dark City which is fantastic, has that kind of "cult" community around it. I think a true "cult" film has to embed itself into popular culture - TBL and Withnail have certainly done that.