Tarkovsky was truly one of cinema’s great directors. His films aren’t simple or easy, but they are gorgeous to behold and if you put in the time, you’ll be richly rewarded. As others have mentioned, Stalker and Solaris are his two great sci-fi films, easily on par with Kubrick’s 2001. For history buffs, Andrei Rublev is fantastic too.
I definitely think that Solaris is a bit of an underrated gem. It's a very long movie that I can't watch all the time, but occasionally when I'm in the mood for a slow, cerebral movie I pop that one on.
If you want a more "abridged" version of it, "Memories" by Satoshi Kon is also pretty great. It's unique enough to be its own thing, so I'm not accusing it of being a "rip off" or anything, but it has similar themes about how memories can influence our perception of people, and it's shorter.
EDIT: Sorry, Memories is the name of the anthology movie. Magnetic Rose is Kon's segment.
