I voted Al.
Brown is averaging more points, more efficiently than Tatum but Tatum draws a lot more attention and has been better defensively. Brown is turning it over less but Tatum is averaging a lot more assists.
Al has been incredible at everything, though. 13-9-3.5 with 2.5 blocks. His shooting splits are insane (.607/.636/.833) and his defense has been truly amazing. He's protecting the rim (don't sleep on those 2.5 blocks) and picking up Jimmy Butler at the top of the key. And that's without even acknowledging the steady leadership he's providing.
It's close enough that if one of Horford, Brown, Tatum, or Timelord has a big close-out game that could decide who wins it but so far in the series, I think it's Al.