Who is the 1 director Leo's legacy would most benefit from?

Honestly, I don't think any would really augment his legacy.

Similar to what Good God said working with an international auteur or the domestic equivalent like Lynch or PTA would give him credibility with the hardcore arthouse crowd, which is the one thing his career doesn't quite have. Fincher and Mann are the kind of thing he generally does so that wouldn't really change his legacy at all.

Him doing a comedic role in a Wes Anderson movie would be a lot of fun though, something goofy like Fiennes in Grand Budapest. I wish he'd do comedy more, his comparatively comedic work are his best performances.

Huh? I can't think of a single film Leo has done that is "like Mann."

To me, something like a Mann film feels like the exact thing I'd like to see in his filmography. Leo at his best is a very good maximalist actor (and yes, he is at his absolute top-form when he's doing comedy), but we've never seen him give the type of innate, internalized, sort of spontaneous and very emotional performances that Mann elicits from his actors. It's the sort of thing I don't think we've ever seen Leo pull off, and I think at this point he really needs something like that to truly cement himself as a great versatile actor. 

Considering the most common criticism I read on-line about DiCaprio's filmography choices is that he's too tame, justing intercalating Scorsese with another white man who directs safe prestige pictures. Which, even if I love Leo and his movies, it's true. His choices are somewhat predictable and considering he usually plays the masculine, troubled leading man, it might seem too "been-there, done-that" to some. It feels like that to me, sometimes.

I think the project he let go of that I think is most regrettable was Danny Boyle's Steve Jobs (which he ironically passed on to do The Revenannt, if I recall correctly). Seeing how praised Steve Jobs was and how great Fassbender was in it, it's safe to say that could also be DiCaprio's ticket to the Oscar trophy. And it would've been a more interesting one because it'd be a character study, something he doesn't do, something that relies more on his acting chops and abillity to create a character rather than the work of a fantastic director.

Going back to topic, as it's been said in this thread, big names like Mann, the Coens, Fincher, PTA and etc could all play to Leo's strenghts and make hits. Yes, they would. But what would them add to his filmography? In which way would Leo's career benefit from those collaborations? They would all likely feel like his work with Eastwood, Iñarritu or Nolan: prestige, but also safe, not so different from what you'd expect. Because of this, I really, really think he should work with someone like Lynne Ramsay or Cronenberg, who can make the intense, aggressive kind of movies DiCaprio is good at, but also make it dangerous, unpredictable, really gritty and bloody. It'd have his appeal, but without being a studio-approved prestige pic, and instead could get the grassroot passion the so-called snobs like.

There's other things I'd like to see him do, like a comedy, but out of the most realistic things, these could be seem as a turn-around, remarkable choice in his career: a character study like Steve Jobs, or a Lynne Ramsay joint.

Edit: how could I forget the Safdies? So, my final answer is: Ramsay, Cronenberg and the Safdies.

Leo is not the pick me, in this situation. For Leo, it is more about the role. I would like to see him do a different genre and stay away from the biopics for a couple of films.

At this point, the cherry on top for Leo's career would be working with the Coens.

someone w indie street cred

I mean, if there's been a constant criticism of DiCaprio's career, it's that he doesn't take risks. I've always been kind of split on that and grown to be on the side that he doesn't have to given his status/stature. However, it's almost certainly why a certain portion of the snobby critics have never entirely warmed to him (which is probably also why he's never won a major critics award). I'm not sure if any of these directors would necessarily benefit him with that contingent. Depends on the type of project. Now does he care about winning the approval/affection of the snobs and/or winning a major critics award? I have no idea, but we're talking about something someone with basically everything would benefit from, so...

Spielberg is kinda washed tbh and I highly doubt that would help him with the critical snobs, who remain the only portion of filmgoers who are not unanimously affectionate towards him. He should work with a young arty director on the rise. Sort of like Cruise did with PTA.

What would you guys think of DiCaprio working with Marielle Heller? She is just tremendous with actors, and I'd love to seem him work with a director with a lighter touch.