There were many great and iconic movies from the 1980's but in my opinion, for whatever reason, a number of really good movies just don't seem to get their due. Your list may differ, Movies ranked by year they came out (not on how underrated they are):
1. Hopscotch (1980) - Walter Matthau as a CIA agent forced to retire by incompetent bosses. He gets his revenge by writing a tell-all book that the CIA and the KGB want to stop from getting published. The very definition of underrated.
2. Excalibur (1981) - stylized retelling of the story of King Arthur and Merlin.
3. Witness (1985) - Harrison Ford as Philadelphia detective forced to go undercover and on the run in Amish country.
4. Silverado (1985) - A Western loaded with actors who turned into stars (Kevin Costner, Kevin Kline, Scott Glenn, Danny Glover plus a brief role by John Cleese). No idea why this movie hasn't gotten more attention over the years.
5. A Fish Called Wanda (1988) - A star studded comedy (Michael Palin, John Cleese, Kevin Kline and Jamie Lee Curtis). This movie did eventually spawn a sequel but I still think the original doesn't get the love it deserves.
As an aside the reason for this list came from two things; one I saw a list of the Top 20 movies of the 1980's and 90% of them were movies I wouldn't watch a second time and I was also thinking of the underrated Steve Martin movie LA Story (which I thought came out in the 80's but actually was released in 1991).