I think dubs for the older anime are better than newer ones (that being said i do always watch a dub first, so i guess I might not be the best to answer this). For older anime think things like Cowboy Bebop, Dragonball Z, Death Note, most animes before 2007, I always watch Fullmetal Alchemist in English, it's just better in my opinion.
FullMetal Alchemist and FullMetal Alchemist: Brotherhood both have great dubs. I did watch some of with its original voice acting, and I preferred the dubs.
Another dub I enjoyed was Your Lie in April. It was good! And the voice actors did a great job of pronouncing the characters names; something that annoys me in dubs is when a character with a Japanese name gets their name pronounced wrong. But Your Lie in April was good at NOT doing that!
Well best English dub..hmm for me for the best it has to be:
1.Black Butler
2.Lucky Star
3.Fairy Tail
4.Fullmental Alchemist (( I forgot if it's Fullmetal or Fullmental))
5.Death Note
6.Naruto
7.Noragami
8.Your Lie in April
9.Pokemon
And
10.Sailor Moon
One of the best dubs I've ever heard is for Black Lagoon. The characters all sound exactly as you would expect and the dialogue sounds real. It's one of those anime I couldn't imagine watching in sub, just because the dub is so good.
FullMetal Alchemist/Brotherhood, Black Butler, and Death Note also have amazing dubs. ^^
Some great dubs that I've heard are Black Lagoon, Black Butler, Death Note, Samurai Champloo, Cowboy Bebop, and Attack on Titan. But my personal favorite is Panty and Stocking with Garterbelt. The cast all sounds like they're having fun and I just love hearing them.
Blood Lad, Soul Eater, Death Note, Hetalia, and Ouran Highschool Host Club all have amazing voice actors. They're great with conveying emotion through their voices without making it too over the top.
And I think Kenshin's voice actor does an amazing job in Rurouni Kenshin. The rest can be kinda cheesy, though. :T
For this I'll use my top ten favorite anime series all of which are dubbed and I've enjoyed said dubs immensely.
Gundam Wing
Fairy Tail
Magic Knight Rayearth
Gun X Sword
Outlaw Star
Flame of Recca
G Gundam
Yu Yu Hakusho
Cat Planet Cuties
Vision of Escaflowne (dvd ocean dub)
with this last series I recommend the ocean dub that Bandai Entertainment released on dvd because the casting was a better fit and had better chemistry by comparision to Funimation's
