Duke McKenzie, because he brings absolutely nothing to every broadcast, closely followed by John Rawling for similar reasons and he also has that 'love the sound of my own voice' smugness about him. These two together are unbearable.
I used to hate the sound of Adam Smith for that silly overenthusiastic voice he puts on all the time, trying way too hard to sound into it. The absolute worst I've ever heard though was Spencer Fearon. He did one show on Boxnation and man, it was truly awful.
I must be the only person around who actually likes Jim Watt's commentary haha, always have, and I also used to like the pairing of Ian Darke and Glenn McCrory when they covered all the big fights on Sky. The best in the business though is Al Bernstein IMO, and the best on HBO is Roy Jones.