Solaris
When man is happy the meaning of life and other eternal themes rarely intrest him.
Solaris takes place on a station situated on a planet which is apparently alive. Even though this is a Sci-fi film it's not your traditional Sci-fi. The first forty minutes are a brief explanation about the planet. The rest of the 2 hours don't really care about explaining the sentient planet setting of the film, instead it explores death, grief, love. The major theme of this film is the unwillingness to let go, the protagonist would rather love a plastic copy of his wifi instead of letting go. The ending of Solaris is one of the greatest endings ever.
I can't wait to watch more of Tarkovsky's work!