Best sleeper movie, an artistic journey...

This film is completely unHollywood.
This film is about as far away from Hollywood as one can get.

It's filmed entirely in black and white, beautiful crisp, clear images
that are highly artistic. The crispness of the film adds to the surreal--yet ultra real quality of the movie. It's how the world looks when you stay up all night (partying or otherwise) and walk into the early morning. Everything is detailed and enhanced, where your eyes see more than your mind does.

The film catalogs one "normal" man's final journey in life. His final walk comes at a time when he decides to accept a departure from his routine existence and try to connect with the unusual circumstance life has placed him in.

This movie is an artistic experience.
I promise you, this one will stay with you for a long time.
It also exemplifies one of the most difficult concepts a person must face, the fact that life owes us nothing--it is a gift,

Cobra Verde (Werner Herzog/Klaus Kinski) and Fitzcarraldo (same) Simply stunning cinematography, story, and acting.
Totally un-hollywood, as you say.

Filmed in 24 countries around the world in 70 mm.

It's a series of images of nature, humans, animals set to a wonderful sound track - no dialogue.

Directed by Michael Fricke. Absolutely jaw-dropping cinematography, editing, and audio

Rough translation of Baraka : "Essence of life"
So...what is your sleeper movie? I might submit a little known, recently produced gem called 'The Woman Chaser' as one which fits all your qualities. Perhaps even 'Dersu Uzala', one of Kurosawa's most poignant and tangible efforts. Or even 'Woman in the Dunes' if we stretched the analogy a bit. As pointed out above, Herzog's films capture a certain quality which alludes to the atmosphere that you describe (although they are not in B & W).
The movie I am refering to is DEAD MAN with Johnny Depp that is the one described in my post. An Oops would be in order here. Interesting responses though, considering I mangled the original post:o)
Aguirre, Wrath of God IS similiar in many ways. It is ultra detailed, crisp, and it follows a journey into the unknown. I happen to like and admire Dead Man more, though. I found it more captivating.

I don`t know if these are considered sleepers,but some underated movies I enjoy are:

Used Cars
4 Dogs Playing Poker
The Emerald Forest
Southern Comfort
I will second Baraka also Mindwalk (all dialogue) and Miracle Mile great movie about a man who answers a public phone and what he hears. SEE WHAT HAPPENS FROM THERE. I beleve it was released in 1989.
I really liked "City of Lost Childrens" from the guys who made "Delicatessen" and "Amelie". Great cinematography and great visual effects.
"Oasis"...Korea.... Shockingly and yet evocative movie about a quadroplegic woman and a slightly autistic man (just check the theme.) Beautiful and its own ways. Acting here makes all Hollywood movies sound like a cruel joke.

"The Color of Paradise" and any other Iranian/Persian movie. All of them are EXCELLENT. The art of storytelling with movement.

"Spirited Away"....animation....the imagination of the director, forgot his name, makes me wonder whether such person can exist and also be a person.

"Chilseowon"....Korea..... winner of the 2002 Cannes for best director. Life of a controversial painter in a period political upheaval. Great insight at the traditions of the less known Hermit Kingdom as well as excellent dialogue, cinematography.

"HATE"....a black and white French movie about the rough Unfrench innercity lives of three friends. Shocking, makes you think whether the inner cities in the US are really THAT BAD. It will shock you. Funny and sad.

"Failan"...Korea...2001...can a relationship like this exist? Talking about love and the power of memory. IT WILL MAKE YOU WEEP.

Darkstar, heh.
"Wings of Desire" & I second "Color of Paradise". That movie just knocked me out, along with every other Iranian film I've seen.
Central Station- very powerful film. Baraka will make you happy that you paid big $ for your gear.
How about Godard's "Vivre Sa Vie" translation: "My Life To Live". French w/subtitles, 1962, b&w. This film has all the qualities that Mrspiggy lists. It is an extraordinary work of art!
Gummo. Very disturbing.