Not sure all of the movies mentioned could be classified as 'underrated'.

My vote goes to The 'Burbs - underappreciated in its day but now accepted as a cult classic. Basically a late 80s comedy vehicle for Tom Hanks but the real stars are his two neighbours - a paranoid Vietnam vet played by Bruce Dern and a man-child played by the late Canadian comedian Rick Ducommun. They spend the best part of the movie trying to convince Hanks' straight laced everyman that his new neighbours are murderous devil worshippers. Classic 80s comedy at its best.