Wow. That's kinda hard to say for me. I've seen a lot that have good and bad dubbing, plus a lot that I've only seen in Japanese.
Personally, I think some of the best dubbing I've ever seen was Blue Seed, which is a part of the reason why I love it so much.
I also thought the Slayers OVA's were done nicely, but they also had some of the worst in them; it just depended on where you're listening at.
That could also be said about the Ranma 1/2 series and OVA's. (I can't say anything baout the movies because I've only seen one of them & it was in Japanese).
Naruto was done well, I suppose, but that "Believe it!" line gets really old really fast.
I've seen people comment on Sailor Moon; I've only seen one of the movies in Japanese, and didn't care for it, from any perspective.
I will agree with kimiski that Tenchi Muyo was done well. I have no complaints.
Sister Princess, for the most part, was done pretty well, too. Although, I didn't like the characters that attempted foreign accents. One was obviously French & I didn't care for it (which is a shame, because it was Tiffany Grant, one of my favorite English dubbers), but I can't make out the other one, (I think it's suppose to be English).