I wouldn't mind taking a shot at this one:
The worst commentator that I have heard so far is (and yes... let's beat him with a piniata stick) is
Sid Waddell (If I'm not mistaken, he was the one that screamed out on the Reyes vs. Yang: "WHAT'S THIS??? A PALM PUSH SHOT!!"... if so, that's the guy). I mean, it's like I just want laugh at him for his sound pitch.
Mitch Laurence has improved so far but I remember that he used to say pretty odd things that would want an amateur audience to absorb from which is why I would somewhat put him in the worst commentator list.
Danny Diliberto should be honorable mention, but due to his age (20, I assume

) I give him props for being such a big ball of knowlege on the mic.
Best ones in my list are: Jerry Forsythe, Mark Wilson, "The Professor" Grady Matthews, Billy Incardona, Freddy the Beard, and (if it ever existed) Minessota Fats cause the man is just too damn funny.