The final act and ending of Rublev is in my opinion the greatest ending in all of film. So brilliant, you can be thinking the rest of the film is slow and tough to decipher, but it pays off as the ending hits so well.

If you liked Andrei Rublev, you should check out Alexei German's adaptation of the Strugatsky brothers "Hard to Be a God." It's insane, gorgeous, disgusting, teetering on the edge of madness, and monumental.

German's "Khrustalyov, My Car!" is also the purest cinematic distilation of paranoia I have ever seen.

Badlands and Days of Heaven are definitely his most conventional films and thus good starting points. Badlands especially is a great film, Days of Heaven is a bit uneven in terms of plot and pacing, but the cinematography is beautiful.

Then you have The Thin Red Line and The New World, which to me feel like a transitional period between the more conventional films and The Tree of Life, which is the first film that is characterized through and through by Malick's extremely divisive style. I personally love The Thin Red Line, but I can see why it's not for everyone. (I would skip The New World.) All later films have a very recognizable style, for which I think The Tree of Life is the best starting point.

Long story short: I'd start with Badlands, then watch The Thin Red Line, then The Tree of Life. If you like the last one, watch any of his later films.

I recommend turning on subtitles for Tree of Life. There's a lot of random whispered voice-overs, and without subs you'll have no idea who is speaking, let alone what they are saying.

Days of Heaven (1978)

Or dive at the deep end and watch Knight of Cups or A Hidden Life. You will either like it or not, frankly I don't think it matters what you'll see first, I love all of his movies even though I didn't understand Thin Red Line when I was 20. But Knight of Cups hit me hard when I was 36.