The Philadelphia Story
His Girl Friday
Some Like it Hot
One, Two, Three
North by Northwest
Butch Cassidy and The Sundance Kid
The Great Dictator
Silver Streak
The Adventures of Robin Hood
Bullitt
Charley Varrick
The Marathon Man
Midnight Run
Don‘t Look Now
West Side Story
The Devil's Brother
Hana-bi
Chinatown
Frantic
Dance of the Vampires [aka The Fearless Vampire Killers]
Taxi Driver
Assault on Precinct 13 (the original one by John Carpenter from 1976)
Day for Night [original title: La Nuit américaine]
All About My Mother [original title: Todo sobre mi madre]
The Great Silence [original title: Il grande silenzio]
The Bridge [original title: Die Brücke]

... to name a few movies which I love despite not being underrated at all (in fact most of them are acclaimed classics in their respective genres).

Let me know if you want me to list more, I have favorite movies across all decades, several countries and most genres.

Unico and the island of magic. seriously, cute and scary movie!

A. I. - Artificial Intelligence (2001, IMDb 7.1), my overall favourite movie. Spielberg and Kubrick, but still underrated.

Tales from Earthsea (2006, IMDb 6.5) - Studio Ghibli, Goro Miyazaki Anime, but still underrated.

Hellraiser (1987, IMDb 7.1) - Clive Barker. Best brutal horror movie with amazing special effects (no CGI junk), or Brain Dead (1990, IMDb 6.1) . Best horror movies were made in the 1980's.

I just watched In the Heart of the Sea.

Right now I have no idea why it was so panned by critics and flopped so hard, the movie got me hooked from start to finish, even Chris Hermsworth's acting was surprisingly really good.

In terms of underrated films I hear nothing about today, it's War Horse (2011)

It's a WW1 film, in a horse's perspective. What's not to love?