Yeah, Solaris is my favourite because I can be a total sci-fi nerd sometimes, and I actually read the novel when I was a kid before I ever saw the movie, and then the film was my first Tarkovsky so it has a special place for me.
Then probably Andrei Rublev, because it's just a masterful piece of film craft, and so epic in scope. Just one of those movies that sticks with you for so long.
Stalker next, which is by far his most "difficult" film out of the three i've seen, but also perhaps his most ambitious and maybe even his most infuential. It can totally turn you off if you're in the wrong mood for it (a little weed helps!).
The separation between the three is pretty superficial though, as on certain days they could all be my favourite.
Those are the only three I've seen, but I own them all and have enjoyed them many times. I always keep meaning to explore the rest but I don't get around to it for whatever reason.