Generally I prefer subs by a mile but there are a couple examples where the dubbing has imo surpased the sub.

The series I feel that this is most evident in is Those Who Hunt Elves where I found the changes to the script for the dubbing made the series much funnier then the subbed version.

Also while I wouldn't exactly say the dub was better I feel the Berserk dub was very well done and I would recommend that people atleast give it a try.

On the other side of this I feel certain shows have been destroyed by the dub to the point I can no longer enjoy the series. For me this includes Kenshin/Love Hina where all the same voice actors are used for both and expecially the random southern voices really effected the series for myself while I love both of those series in the subbed form. A example of a absolute abomination of a series being dubbed is One Piece but thats also a example for everything wrong with changing a anime in general. Finally as opposed to Love Hina and Kenshin where I hated the voice actors in both subs I found the Inu Yasha dub aweful because it reused all the same voice actors in the same roles as Ranma 1/2 and this mad it unwatchable for me. Note this had nothing to do with disliking the voice actors just the abuse of reusing everyone in a series that ends up seeming identical to another with the way they did this.