On the surface it's easy to say to Al Horford, however I think we could see all of these players within a few points of each other.  Horford may even be relied upon more as a passer this year with so many scorers on the team. 

Here are each candidates last season's PPG.

Al Horford 14 PPG
Marcus Smart 10.6 PPG
Jaylen Brown 6.6. PPG
Jason Tatum N/A
Terry Rozier 5.5 PPG
Marcus Morris 14 PPG

I think we could see a downtick with Horford's scoring maybe to around 13 PPG.  Smart has looked like a new player so maybe at least 12 PPG.  Brown is taking over Bradley's starting spot so we could see a nice jump maybe to over 10 PPG.  Tatum who knows being a rookie but it wouldn't surprise me somewhere around 10 PPG.  Rozier has looked awesome in preseason but I don't see where much of the scoring chances are going to come from.  Maybe 8 PPG.

I'm going with Morris.  If you check his games logs from last year.  Since January 1st he's had 10 games at least 20 points (2 others were at 19), and 2 of those games he scored 36 and 37 points.  In a better offense with more spacing he might stay at least 14 PPG.   

It's going to be interesting.  I don't think it's clear cut it will be Horford but he will probably lead this vote.