In order of threat level to Barry:
1. Reverse Flash, even IF my idea that he cannot kill Barry due to the timestream needing Barry is correct, Reverse Flash can still stalk and terrorize him and easily take down his friends and family. Sometimes a villain that hurts the hero is worse then one that would kill
original Khan in Star Trek 2 stated to Kirk how he had hurt him by blasting the Enterprise and then leaving him stranded on what he thought was a dead planet...."buried alive!...buried alive!" Then of course came the greatest wound Khan could give Kirk by causing the death of Spock.
2. Captain Cold: a tactical thinker who appears to be a step ahead of the Flash due to his plans. He planned for their capture in this recent episode and for Flash to have gone public thus changing the "game" as it were.
3. Heatwave: a bit of a goon, not exactly a fast thinker but he's a pyromaniac with a heat gun so that puts him right after Captain Cold on the threat list.
4. Clock King: Flash didn't face him, but given his level of thinking and planning, Clock King could conceivably be a problem for Barry in the future.
5. Captain Boomerang: explosive Boomerangs and razor sharp boomerangs could be a problem for Flash if he can't dodge them.
I would reverse 1 and 2, as RF does not seem to want to kill off Barry right now, while Cold will!