highest ceiling is an interesting way of putting it. my list would go:
Mckinnon is in a great offense not only for his skillset, but just for RBs by itself. he doesnt have much competition to work with, and COULD be a 275 rush attempts with 70 receptions guy. i have him much lower than that, and his TDs are questionable, because… well thats the name of the game with TDs.
Howard is looking at a nagy offense that knows how to use its pieces. if he ends up somehow getting 40 receptions, with that zone rune scheme that fits him so well, he could end up with a very high total of yards and TDs.
CMC is going to get his receptions. his 80 last year alone should tell you that. the concern here is CJ Anderson. he is going to soak up most of the carries. probably around 200 or so. if CMC can get his carries to about 200 as well, his TDs should go up as he is a decent redzone threat with receptions out of the backfield or lined up in the slot.
Mixon has the most talent, but is in the worst spot for it. his ceiling in an offense that would use him like he should be, is RB1 overall. he has the skillset to do it, but he wont. not with marv there. so even though theoretically his ceiling is the highest from a skillset standpoint, his actual production (just like anyone else) is based off of his usage. and i just dont see him getting used enough to be that high of a ceiling. so i think in this offense his ceiling is RB12. breaks off a few runs, catches some more balls, and maybe ends up with 250 carries. i just dont see him as a reliable option, which i know differs from most peoples opinions of him for this season.