Heavyweight - He might be champion, but Cain Velasquez is still the best prospect at HW. He could be scarily good in a few years.
Light Heavyweight - This division is going to be ruled by 2 fighters in the near future. Those fighters are Jon Jones and Phil Davis. Both have fantastic wrestling credentials (more-so Davis) and are improving their all-round MMA game at incredibly fast levels.
Middleweight - At 185 Alan Belcher is the dude I see as most promising. He has ever improving, vicious stand-up, he's exciting to watch and proved with his submissions over Patrick Cote and Denis Kang that he's no mug on the ground either. He seems to have been around the sport a long time, but at only 26 I think we're going to see a lot more from 'The Talent'. An honourable mention also has to go to C.B. Dollaway. His face might be the most punchable at 185, but with wrestling as good as Dollaway's and an ever improving submission game, good luck landing that punch on him.
Welterweight - At 170 Dong Hyun Kim clearly stands out. He looked hugely impressive in his last fight against Sadollah, and other than a split decision loss to Karo Parisyan (which was turned to a NC due to Karo testing positive for steroids), he has an unblemished 13-0 (with 1 draw) MMA record. His next fight against a fellow prospect, Nate Diaz, will be a huge test. Nate possesses a very similar fight style to his brother Nick. He's tall, has rangy boxing with decent power, more than decent takedown defence, and is a brown belt in BJJ under Cesar Gracie. That fight, to me, will show us who the most promising prospect is. I'm personally going to go with Diaz. Honourable mentions also have to go to high level wrestler Johnny Hendricks and impressive youngster Rory MacDonald.
Lightweight - Probably the most stacked division in the world. The UFC 155lb division houses some of the best fighters in the world, and hence, some of the most promising prospects. It's a relative newcomer to the UFC I'm most excited about however, that being Charles Oliveira. As soon as I saw his first fight, against Darren Elkins, where he regained composure immediately after being slammed, to pull out a slick triangle choke I knew we had a prospect on our hands. He mixes that vicious brand of Jiu Jitsu with surprisingly explosive strikes. He mixes up his punches and kicks superbly and in his last fight against Escudero put on an absolute clinic before finishing the fight via a choke. Oliveira's not the only prospect though, honourable mentions also have to go to Evan Dunham, Cole Miller and Jim Miller.