I haven't seen too much of him during his prime, considering I'm not really interested in wrestling from the early 90s and before, but I've seen everything (WWE-wise) from Evolution-on, and a little before that. On the microphone, he's obviously great. I wouldn't say one of the all-time best talkers, like others may, but he was still very good in that respect. In the ring, he was still able to go and put on great matches when I was able to watch him wrestle, so I'd imagine he must've been really damn good in his prime.
Based on what I've seen of Flair, I'd give him an 8.5.