Mine would have to be, in no specific order:
The Rock - the guy could really cut a promo mixed with cheap pops (finally the rock has come back...), and cutting comments. Never failed to raise a smile.
JBL - Gets real good heat with his cheap racist pops and never stuck for words. So natural.
Edge - As with JBL, very natural and likes to make reference to real life events to gain heat, which works. Recent exchanges with HHH & Cena some of the best mic work seen for some time.
HHH - Whether it be as heel or face, always interesting.
Superstar Billy Graham - The original. Mic work wouldnt be as important as it is in wrestling today without him.