Thinking about it a little longer, I missed a few important voice actors who really need to be on this list:
Satsuki Yukino, the voice of: Misae Sagara, a side character in Clannad, #1. Shion/Mion Sonozaki, the twin heroines of Higurashi, #5. Kagome Higurashi, the main heroine of Inuyasha, #32. Kaname Chidori, the main heroine of Full Metal Panic, #22, and many other roles.
Kana Hanazawa, the voice of: Kobato Hanato, the heroine of Kobato, #20. Nessa from Fractale. Tenshi/Kanade Tachibana, the rival heroine of Angel Beats, #16. Nadeko Sendoku, the coolest girl in Bakemonogatari, #75. Mikan Yuki, the cute little sister of To Love-Ru. Ruri Gokou (Kuroneko), the rival heroine of Oreimo, #42. Shiori Shiomiya, the library love interest from The World God Only Knows (who gives the greatest speech ever about books), #28, Charles Dunois, the coolest girl in Infinite Stratos, Cleo Saburafu, a fascinating girl in Break Blade, #71, and many other leading roles. She's making appearances in this spring season's Deadman Wonderland, Steins;Gate, Blue Exorcist and Dog Days, so be sure not to miss her. She's working hard to entertain the whole world with her voice, and definitely deserves credit for her contribution.
Norio Wakamoto, the voice of: Cell, from Dragonball, #8. Chiyochichi, the freaky cat from Azumanga Daioh, the narrator for Hayate no Gotoku, #31, Charles vi Brittania, the villain from Code Geass, #3. Woodchuck, from Record of Lodoss War, #25. Biagio Busoni, a villain in Index, #14, and a million other roles. His distinctive voice that feels like it is always giving an epic speech lends a grandeur to any anime he's put in. Even though the man is 66 years old, he's a leading character in Dog Days, a currently airing spring anime. That's dedication. Hopefully he'll be available to narrate more Hayate, when the promised third season begins!
There are many other great voice actors, but I want the list to be selective, so I'll add just these three to my hall of fame for now. Looking back, it's amazing to note that Clannad and Higurashi managed to include yet another of the best voice actresses in the business (Satsuki Yukino). It's no wonder they ended up being two of the best animes of all time.
Even though I look at how many important roles in good series someone has played to decide if they're truly great, that's not the decisive factor, so I'm not just adding every voice actor who, say, worked on Clannad, simply because they were in Clannad. It's because their voices were marvelous, memorable, and evocative when I listened to them too. They demand attention the moment they start saying their lines, their *presence* is just more felt. That is what makes them so great, not who they are playing, but how well they play their characters.
It's also no wonder girls dominate this list. Girls have larger vocal ranges, anime is largely for boys, which means cute girl voices are a premium, and the book of Nature evolved women to have more pleasant voices, because women were designed to be pleasing and attractive to men -- just as male birds evolved to become excellent singers to court female birds, and thus have a surfeit of vocal talent compared to the females of their species. It's the exact reverse in humans, girls spent a lot of evolutionary *points* to achieve the wonderful voices they have, and so when it comes to singing or voice acting, they will always dominate over men. Men and women can work hard, men and women can be talented, but only women are designed by God almighty to have beautiful voices. That's just how the cookie crumbles.