This entry's about the best dubs I've heard.
#1: Wings of Honneamise
Manga Entertainment had only just gotten into releasing anime when this title hit the shelves. The movie is considered one of the best anime movies ever produced. It's the movie which put Gainax Productions on the map. Only GP could have made this movie; and unfortunately its critical success inflated their artistic ego to the point that they made Neon Genesis Evangelion.
The dub of Wings of Honneamise is excellent. Manga Entertainment put a lot of time and effort into doing it correctly, and it shows. It's definitely one of the best out there.
#2: El Hazard
In the early days Pioneer was another company which understood that a good dub could make or break a title. El Hazard was one of the series they released by the episode--one episode per tape or laserdisk--and they didn't skimp on production quality.
All the actors could act; they could pronounce the names; and there are few problems with the dub. The first five minutes of the first episode are a bit rocky, but afterwards, the dubbing is consistently good.
#3: Tenchi Muyo!
This is another Pioneer series from the same time as El Hazard. The first six episodes of Tenchi Muyo! were released by the episode and the dubbing never faltered.
#4: Golden Boy
When you watch Golden Boy you have to be ready for Doug Smith's performance as Kintaro Oe, the main character. He makes the dub of this series. Without him, the dub would have been utterly worthless.
This is one of the few cases where the dub is actually better than the sub. Bob Marx (screen name for R. Martin Klein) did much the same for Katsuhiko Jinnai in El Hazard.
#5: Ranma 1/2 (first season)
The first season of Ranma 1/2 is arguably the best season of that show ever produced. It didn't play well in Japan, though. Still, the quality of the dub was first-rate. Viz Communications soon became synonymous with bad dubs, but this was before then.
The first 4 or 5 episodes featured Brigitta Dau as female Ranma, and her performance was perfect. She was replaced by Venus Terzo, who--while capable--just wasn't as good a choice.
#6: Project A-ko
The first offering of the Project A-ko series was an excellent dub, even though the sound quality was less than great. The actors could all act, and the voices were all perfect. The best among them was Denica Fairman, who played B-ko; she had the right voice for a rich girl like her.
Subsequent volumes became the Ranma 1/2 reunion. Venus Terzo did B-ko, Teryl Rothery did A-ko, Cathy Weseluck did C-ko, Robert Smith did a lot of the male voices, etc, etc.
#7: Hand Maid May
If you watch El Hazard, during that rocky first five minutes, Lia Sargent's performance is the worst. She sounds like she's in a junior high school play. After that, her performance improves greatly when her character shows up near the end of episode 1, and it's immaculate when she appears again in episode 3 of that series.
Here, her performance is perfect throughout the series. It never misses a beat.
The other voices meet that standard rather nicely. No mispronunciations, either.
My only complaint is that one of the characters begins to speak in the Japanese Kansai dialect when agitated, and the dub's producers chose to simulate that with a hillbilly accent. That's one of my pet peeves about dubs.
#8: Oh! My Goddess! OVA
Prior to the release of this as a dub, AnimEigo's best success with dubbing had been Those Obnoxious Aliens, their dub of Urusei Yatsura. It was a complete failure and they never produced another volume of it.
Oh! My Goddess! redeemed them. The dub is entertaining in ways the sub is not. It's hard to decide which version to watch, as both sub and dub have their good points and bad points.
My favorite moment in the dub is a scene where Urd has decided to help Keiichi "get together" with Belldandy, and she gives him a bottle of "Urd's Very Own Sunscreen Oil":
"You just rub it on her, and she'll get all hot and steamy," Urd purrs.
"You mean, Belldandy will...?" Keiichi asks, swallowing hard. He imagines Belldandy, all heated up, and he emits this wolf howl.
Urd smacks him on the back and says, "Heel, boy!"
#9: You're Under Arrest! OVA
This is another AnimEigo dub, one released about the same time that the dub of Oh! My Goddess! was. The performances are all excellent, as is the rewriting that is required when you're doing dubs. The subbed version lacks things the dub has, but the same is true in reverse; so this is another series which makes it hard to decide which version to watch.
#10: My Neighbor Totoro (Fox dub)
There was no reason for Fox to do this right at all. I mean, they took a little-known Japanese cartoon movie and brought it over to the US long before most people had heard the name "Hayao Miyazaki". They marketed it to families, not to anime fans; they could have butchered the shit out of it and they still would have gotten their money out of it.
But something happened that you wouldn't expect: they left it alone. They didn't cut a single second from the movie. They did the dub right, 100% right, and didn't mess with it at all.
You can no longer buy the Fox dub, as far as I know.
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You might have noticed that I didn't mention any of the Disney/Buena Vista Video dubs of the Miyazaki films.
While the quality of the actual dubs is first rate--and while Disney was not allowed to cut anything, by contract--they did something almost as bad: they added things.
Mostly what Disney did was to add music and "foley" to otherwise silent scenes. There are sequences, for example, in Kiki's Delivery Service which--in the original--are silent, but in the dub version there is music and other noise.
When Miyazaki made the films, there were parts where he purposely had no sound--places where the lack of sound set the mood of the scene. The silence is meant to emphasize, for example, that nothing is happening on a particular afternoon. Adding music ruins the mood.
These differences show up when you watch the DVDs with the Japanese language track playing. Disney almost released these films without that, particularly Princess Mononoke.
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There are plenty of bad dubs out there. Those Obnoxious Aliens seeme to be one of the worst.
Most of what's wrong with dubs can be seen, in example, here: Strong Bad Email: Japanese Cartoon
A lot of the dubs out there actually sound like that.
In Japan, when an episode is voiced, the actors all sit in a room and the episode is acted out in real time, so the actors in scenes can play off of each other and it all sounds very natural.
In the US, the actors typically go into a sound studio one at a time and record their lines. The actor has to be GOOD or else there is nothing natural-sounding about it. This is why a lot of dubs tend to sound like middle school plays; the actors are trying to act in a vacuum.
...or else there are just a lot of crappy voice actors out there.