Hmm... I'd say I like Klobuchar/Castro best, since I think she's a great candidate (both in the actual genuine "presidential" sense, and in the sleazy, no-good, political strategist sense), and I would to see like some continuity from the Obama administration (assuming the Castro is Julián). Plus, I think this would be a good balance between the "identity" politics and the "not identity" politics (if there's even a difference), since Klobuchar has been very good on workers' issues (as far as I know), and Castro was the Secretary of HUD, and she has legal experience while he has executive (mayoral, but still) experience. Plus, she's a woman and he's Hispanic, and I think those facts would prove to be very important as history looks back on our time.
I really like Brown/Harris and Booker/Klobuchar as well. These are all very well-balanced tickets imo, so I would be very happy to campaign for any of them once 2020 rolls around
Agreed, though I wouldn't be that happy with the other two tickets- Brown is too anti-free trade and I dislike Booker.