Although I have a preference for subtitles, I saw the majority of the Tenchi series dubbed. As far as dubs go, Tenchi Muyo is probably one of the best done that I've seen. Granted that the first and second episodes of the OAV had some problems, but not long after that all the problems got ironed out. The dub actually didn't detract much from the series at all. Even with all that said about the dub, I still think the subtitled version to be better. Of course I have my opinions on individual characters. While I think the majority of the voice acting was well done, there's still a few things that stand out.
Ryoko - Hmm..... if you would have shown me the original Tenchi series, then let me listen to the English voice actor, I would have said your nuts. Of course now I think a bit differently. What can I say? The English voice actor is perfect. Even though her voice is completely different from the Japanese voice, it still fits perfectly. Such a diverse character as Ryoko, and very rarely even a slip in the voice (we'll overlook some parts of the first OAV episode). Of course the dub voice actress is simply living up to the Japanese voice actress who is also VERY good. Either way, Ryoko sounds terrific.
Aeka - Well, in the beginning I REALLY didn't like Aeka's voice, but I think I've more or less built up a tolerance now. Her voice actress is well suited for calm moments, but is slightly lacking when Aeka gets angry. I do of course like Aeka's Japanese voice actress, who certainly is better at sounding frustrated.
Mihoshi - A little reversal here, I thought Mihoshi's Japanese voice actor sucked. Not so much because of the talent, but because I found the voice itself to be irritating. I didn't think Mihoshi's voice within itself had to sound like that of an idiot. The English voice got the point across with a little skill in the voice acting, and of course with Mihoshi's dialog. So for once I found that the English voice actress sounded better.