Thinking about things since my last answer i've changed my opinions a bit.
In the early campaigns, the MIG 19 is the big threat across the board on the Red side. It has it all over the F-100 in '62 and is still quite a threat vs F-4s in a close in fight. It's speed also makes it harder to disengage from compared to a 17. Speaking of the Mig 17, if you ever get low and slow with one around you are as good as gone.
Call me a bit blase, but the MIG 23 i never consider a threat in a fight. The only threatening thing about them is SARM shots as one closes in or tries to disengage.
In later campaigns the SU27 has a bit of an edge over the MIG 29 but both are formidable aircraft.