Now that we have this wonderful Anime Voices 4.0 resource list, it would be a shame not to apply it to my best anime rankings list and see what happens. If we run a cross-check on good series that also had good voice actors in prominent roles, what do we get?
The answer should become clear quickly. The series with the best voice acting are:
Zettai Bouei Leviathan,
To Love ru,
Baka to Test,
Akaneiro ni Somaru Saka,
Sword Art Online,
The World God Only Knows,
Full Metal Panic,
Zero no Tsukaima,
Spice and Wolf,
Shakugan no Shana,
and Dragon Ball
in roughly that order. The top series are overflowing with voice talent with virtually every character being a superstar. Lower down there tends to be 3 or 4 top voices occupying main character roles, and at the bottom there's series with at least 2 top voices inhabiting 2 main characters. Series with more characters are more likely to have more good voice actors, so it's hard to precisely score the percentage chance you'll be listening to an effervescent siren per second while watching one of these good anime series, but basically the odds are pretty good from top to bottom. An example of a series with just two main characters, both of whom are top anime voices, creating a sonic utopia by only talking to each other would be Spice and Wolf. An example of a series with insane amounts of good voice actors, but so many main characters that you'll likely be listening to a non-superstar regardless of this fact, is Pretty Cure. To be safe, if you just want to listen to good voices, choose a series from K-On or above and you'll most likely be listening to someone amazing.
The most interesting dynamic about voice actors is, do the voice actors make the story good, or do the good characters in the story allow the voice actors to shine? Which is the chicken, and which is the egg? My answer is the voice actors. A story can be good as a book or manga, it doesn't need good voice acting to be appealing. But anime is different, without good voice actors, no matter how good the plot or visuals, the anime will seem terrible. And if the voice acting is good enough, even the worst imaginable series end up watchable (or listenable, as it were). If the voice acting is good, the anime will be good. Naturally, voice actors are inspired by their characters who give their voices directions to move in. Just like singers who are given a theme to sing about end up singing much more beautiful songs than they would be able to compose if simply left to their own devices. But I don't think anyone would have realized how great Nanoha was without Yukari Tamura, Code Geass would have been nothing without Lelouch's inspirational voice versus Charles' ominous voice, and Clannad couldn't have carried the emotions it had without perfect voice actors for Tomoya Okizaki and Nagisa Furukawa. Toradora wouldn't have been such a riveting romance story except that all three girls the main lead had to choose between had godlike voices to go alongside their beautiful anime visuals.
This is why anime absolutely has to be watched subbed, the way God intended. Dubbed anime does not have the right voice acting for the job, and the original creators did not cast them for the roles they dub. Subbed anime straight from Japan has characters who intimately understand the story that's told in their native language, and were hand-picked by the creators of the story to fulfill those character voices. The difference is so high that dubbed anime, no matter how good the source, is practically unwatchably bad. You have to understand that voice acting in Japan is a prestigious, competitive job that tens of thousands of people attempt to become every year. There are entire colleges to train seiyuu in. From that hellish level of competition, only the best of the best are allowed to play Girl A in completely obscure roles. And yet there are some voices who are given the plum spot to virtually every series -- like Kana Hanazawa, Rie Kugimiya, and Yukari Tamura. What does this say about them? They are the most beautiful voices known to man, honed by the sharpest grindstone known to man, the Japanese art industry. They're the gold medalists in the Olympics of voice acting. When you take that level of skill and reduce it down to a low budget dubbing company that caters to a few dozen fans outside of Japan, and anyone who walks into the studio can get the voice acting role the day after, what did you think would happen?
Anime appreciation starts with watching things subbed. After that, listening closely enough to discern individual famous voices from each new anime series is the next step. Memorizing the names of all the seiyuu as well as their distinct vocal patterns is the last step. Only then can you call yourself a true fan. Let the journey begin!