If we are going purely by Key works, my favorites would be Tomoko Kawakami.
Tomoko voiced part of the dynamic duo, Sayuri. She also did Misuzu in AIR, and the sp00ky girl from Clannad. In case you can’t tell from these roles, she’s pretty much the goddess of “wow… that’s the voice of a breaking human being.” The amount of emotion she manages to portray is really surprising, no matter how much I hear it. Look at AIR for example. Every line Misuzu speaks is glorious. I don’t even like the character, but the voice is sad and yet happy.
It’s pretty much the best thing about AIR.
Harumi Sakurai gets an honourable mention though.
Outside of Key… The easy answer would be Kanahana. Kawai from Kawaicom, Shiro in DMWL, and the best role, Kaminagi. That’s only including characters from my top 30 list. She has sooo much more too, she’s a good singer and even her non-anime content is good.
Another easy answer would be Yui Horie. I skipped so many episodes of the Litbus anime that I can’t be agree or disagree with @Pik3rob’s comments, however I do know her roles outside of Key. I don’t think I know anyone who has voiced more characters than her actually. She’s been a prominent voice actor since I got into the anime scene. My favorite role of her’s would be as Anna. My second favorite would also be Anna. HM then K.
The hard answer would be Maeda Kei (or whatever alias you prefer.) Kara no Shoujo is one of those VNs that you have to replay a lot. For some reason, when skipping through scenes, there are moments where one voiced line carries on through the scene transition. One of these lines is a heart-breaker. The first time you hear it is sad enough, but when you hear it over and over, unable to skip through it, every single time you try and reach that ending… And it’s said so well! The first time you hear it, it is the big moment. It’s the moment that you begin to care for this character who really hasn’t been all that of a factor to the story. You then have to live that moment repeatedly… Each time, the emotion of the line changes… to be more intense than the last.
I don’t know if it’s intended, or a problem with moving the VN over to a different engine during localization without the voices in-mind, but it’s an amazing little quirk that the VN has.
For English voice actors, I agree with @Inuconandoyle in that Crispin is good. It’s very generic, but it’s good. He’s definitely not my favorite though.
Quinton Flynn, hands down. Reno, Axel, Raiden. It’s not often I praise KH or FF for it’s English cast, but man, he has some really good moments.