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.
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.