Videos of individual promos can go here:
http://www.thesquaredcircle.smfnew.com/index.php?topic=73.0 
Going back and looking at past promos, IMO Foley was one of the best. Especially the ECW ones. Even if you aren't completely filled in on the current storyline, there's just something about them. On His 'Greatest Hits & Misses' DVD he says that one of the secrets to a good heel promo - advice given to him by Michael Hayes if I remember correctly - is that the bad guy has to believe he's right and he used legit anger and frustration he had when doing promos.
Of course, guys like The Rock and Michaels are great too. I also remember some amusing promos from mid-late 2001 with Kurt Angle and Stone Cold Steve Austin; Austin and Angle singing to Vince, Angle continually repeating what Austin said etc. Those are two guys who could pull off both funny
and serious.