To be honest, I dont think it is the hit of the year. Figure the speed they were going at, the force of impact wouldnt be as big as some of these other hits the safeties have put on this year because the speed that a WR and the speed a safety are going at before collision. Clowney wasnt at full speed and Smith definitely wasnt going very fast. It looks much worse than it is because Clowney hit him with perfect form and his helmet actually hit just under Smith's helmet. That would knock off anyone's helmet. Thankfully Clowney isnt in the NFL yet or that would have gotten a fine from the ruthless Goodell.
This video is a compilation of hits from college last year and next month the video from this past season will be up.
https://www.youtube.com/watch?v=bv89-0CdEIYMajority of those hits are bigger than Clowney's and I will assume there are hits bigger than Clowney's in terms of how big of a hit they are. It just looks worse because of the helmet coming off which is just the result of the helmet hitting right under it.
I will say though Clowney is a freak and living up to every expectation of him coming out of high school. He is the next Julius Peppers without question. I just hope he doesnt get screwed and get drafted by a 3-4 team. He is a 4-3 defensive end and that is the defense South Carolina runs, he needs to be in a 4-3 system. Too many players get screwed by the system they get put in by changes in coordinators going from 4-3 to 3-4 or vise versa.