I guess where I come in on this is first the process of elimination. Like many have said, Estes in no way is ever playing QB here. Williams and McLautin were both projected QB'a in a run first read option attack. I would be shocked if either plays at QB during their time here. Williams will be a WR after his situation settles and McLaurin is unfortunately a guy who has been put into an unideal situation for him. He's just not a great fit going forward now. Unless he can play another position, doubt we ever see him on the field.
So that leaves the four "obvious" candidates. Morgan, Green, Croft, and Rhoda. Personally, I'd really like to see Morgan RS. No doubt in my mind he's the most talented one of the four but is he ready? Early enrollee helps a bit but still a big task for a true freshman.
Whoever wins out, they are in a good situation in terms of they have two reallly nice running backs to take the load off.
The passing attack this year is going to be pretty simple IMO. I know it's cliche but as the staff settles in with a new system, they're going to try and make the passing game as easy as possible especially since they don't have an obvious starter ready to take control waiting in the wings. It's going to be about simple routes and simple reads and whoever can be the most accurate AND not turn the ball over will eventually win the job.