Tree Of Life is nothing short of a masterpiece IMO. Influential on me personally as my first exposure to how much editing and structure (or lack thereof) build directorial style. It left an impression on me to feel so much for a film that explicitly says so little.
Obligatory mention of that iconic low-angle shot of The Mother floating gracefully across the plains. One of the best of all time.
The Tree of Life is singular to me as a piece of cinema, americana and a meditation on the beauty of life and especially childhood.
When I saw it the first time, I was so awestruck by the breathtaking cinematography and the incredible music, but even more so by the vision of it all. I had simply never seen anything like it.
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.
I’m a huge Malick fan and agree that The New World is his masterpiece. I still remember seeing it in the cinema 20 years ago and almost levitating out of there. Just a beautiful piece of work. I’m glad there’s just about room for Malick somewhere in the film industry.
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.
Obligatory mention of that iconic low-angle shot of The Mother floating gracefully across the plains. One of the best of all time.