Love him or hate him, CM Punk is the best on the mic. As pointed out above, Jericho is still good, but his current character is stale and thus becoming boring. CM Punk always cuts great promos. Loved the one he did at Night of Champions in Chicago. Then I happened to catch him on NXT last week as an announcer, and I actually watched an entire episode of Divas NXT because he made it so entertaining. And he claimed he would be back next week, so I'll be tuning in to NXT again Tuesday night.
Punk's material is always great, but the main thing is his believability is through the roof. Rumor has it that there isn't much room for improvisation in the WWE these days and things are scripted to the letter, and many guys are obviously struggling with that, and making it very obvious how scripted their lines are. They sound incredibly fake, just the typical "bad actor" wrestlers. CM Punk sounds so authentic. I guess he benefits from the fact that his gimmick is really who he is. Other guys might be working with material that isn't quite so natural to them.
Whatever the reasons, Punk is miles ahead of anyone right now. Despite being a heel and pretty much saying the same things every time, he doesn't get boring like Jericho and others.