On the real though, I think the best commentators are either veteran and/or retired fighters or highly respected trainers. On HBO, when it's Bernard Hopkins or Emmanuel Steward doing commentary, it's very enjoyable and I learn alot. Teddy Atlas is another that comes to mind.
In the UFC, the best one who fits this mould would be Randy Couture. The fact that he's knowledgeable and brings a subtle dry humor (Oh, the double head bounce...*heh*) that comes out, well, naturally, makes his commentary a joy to listen to.
__________________ Israel Vazquez....Super Bantamweight Champ....Super Heavyweight Heart
This man is my hero.