Many seiyuus are popular, but only some are seen as having real ability as seiyuus. Those are known as 実力 jitsuryoku "real strength" or "real ability" seiyuus.
Other seiyuus can be popular for the sound of their voice or for their personality, beauty or singing and dancing.
Here is a list from 2channel of someone's idea of who the really capable seiyuus are, starting with those born in 1993 (now 18-19 years old) and going up to those born in 1982 (now 29-30 years old):
1993 Ishihara Kaori, Seto Asami
1991 Kotobuki Minako, Hayami Saori, Yuuki Aoi
1990 Tomatsu Haruka
1989 Uchida Maaya, Omigawa Chiaki, Hanazawa Kana
1988 Komatsu Mikako, Ise Mariya, Akesaka Satomi
1987 Kayano Ai, Takamori Natsumi, Tamura Mutsumi, Kitamura Eri
1986 Toyosaki Aki, Koshimizu Ami, Sanpei Yuuko, Itou Kanae, Yahagi Sayuri, Satou Satomi
1985 Takagaki Ayahi, Sawashiro Miiyuki, Nazuka Kaori, Inoue Marina, Hikasa Youko
1983 Katou Emiri, Asumi Kana
－－－－－－－30 years old－－－－－－－－－－
1982 Fukuen Misato, Shiraisi Ryouko, Fujimura Ayumi, Watanabe Akeno, Kobayashi Yuu, Saitou Momoko
note: I say "born in," but I think these are actually arranged by school cohort, which goes from the beginning of April in one year to the end of March the next.
This doesn't seem to me like a perfect list, but it's pretty good. I'd now include Hidaka Rina (1994), and maybe exclude Kotobuki Minako (much as I like her). And I know plenty of people will go "whaat?" at seeing Omigawa Chiaki there, but I would still include her.