Monsoon and Heenan were aweshome. Monsoon also worked well with Jesse Ventura. when Monsoon would occasionally slip into lala land and say some bullshit, Jesse would always call him on it. it was great because those were some of the few times Monsoon had no response. but he and Heenan were gold.
as for currently the best: Joey F'N Styles.
because he can call everything. he can do play by play, he can analyze, he can be as serious or funny as needed. he's great timing. and this is the key: he helps push the story that's going on in the ring. plus, he's got the best catchphrase for an announcer ever: "OH MY GOD!!!!!!!!!!!!"
Mike Tenay's grasp and knowledge of wrestling history is unparalleled, but he can be a little bland at times. I find he works best in the three man announce format, generally. He's having to carry Don West right now, which doesn't help his work.