They're not exactly underrated, but a couple of oldies I've re-watched lately that faded from memory but are nonetheless still powerful dramas are King Rat and Who's Afraid Of Virginia Woolf? Both have an actor that was seriously underrated in George Segal, and Woolf has Burton and Taylor at the peak of their acting chops.
Others might include:
The Pope Of Greenwich Village - Eric Roberts has always been a much better actor than his more famous sister Julia, IMHO.
Galaxy Quest - Little sci-fi comedy gem. Alan Rickman as a former Shakespearean actor reduced to doing Trekkie-like conventions is hilarious.
Clash Of The Titans - The original, not that remake shite.
Mutiny On The Bounty - The 1962 version with Brando as Fletcher Christian. Nominated for seven Academy Awards but didn't win a single one, which was a travesty.
The Four Seasons - Comedy/drama in which four couples who take all their vacations together face the insecurities of growing older. Carol Burnett is great.
1941 - Almost universally hated by critics, but one of my all-time favorite comedies. Beware, there are two versions out there. One has a bunch of mindlessly unfunny crap but the second is more tightly edited and that makes all the difference. It's a crapshoot which one you might see/buy.
The Chronicles Of Riddick - Vin Diesel can't act for shit, but when you can kill a guy with a tin cup and soothe giant alien lion-like creatures, who needs it? Epic sci-fi villains in the form of the Necromongers.
Tom Horn - Pretty good western with Steve McQueen in top form. He died just a few months after its release.
