Companies? Wow that one is hard because I think even companies that do a great job can drop the ball from time to time.
For instance with Bandai I think that the Witch Hunter Robin dub is on the money...but .hack//sign really sucks. With ADV I think that Angelic Layer is pretty darn bad (though it grows on you) and Azumanga Daioh is really god. So for me it's hard to pick companies
I think from shows one of the best is Cowboy Bebop and one of the worst dubbed is again as mentioned above .hack//sign.