My own personal bias aside, I think both Jonathan Williams and Alex Collins will be effective backs at the next level.
Williams (6'1" 225) is big, fast, and has good hands out of the backfield. He doesnt run with as much power as you would expect to see from a guy that size, but he will easily run through arm tackles, and occasionally lower his shoulder and run over smaller DBs. He reminds me a lot of Knile Davis when he was at Arkansas with a similar size and running style. It's really unfortunate that he will miss this season due to injury.
Collins (5'11" 215) looks much more like a Seahawks back than Williams. He has excellent vision, Marshawn-lite ability to get yards after contact, and is suprisingly elusive in the open field. He doesnt have the same freak athleticism as Williams, but he makes up for it with his vision and power. Last year, he looked sluggish at times, but this offseason he shed ten pounds, and in just a one game sample, he appears to have much better explosiveness. He also has really good balance to seemingly fall forward on every run, a-la Lynch. He is definitely not Marshawn (who is?), but I wouldnt be surprised at all if this is a guy Pete and John target in the next draft if they miss out on Derrick Henry.
It also helps these guys that they run behind one of the best O-lines in the country (doesn't necessarily mean they can't be good without a good line). Bret Bielema was always able to put together dominant O-lines when he was at Wisconsin, and that also seems to be his main priority here at Arkansas. Denver Kirkland (6'5" 340) is a guy to keep an eye on. He was a dominant run-blocker and more than servicable in pass protection at RG last year, but this year he switched over to LT. Im anxious to see if he is able to handle speed rushers off the edge at his size.