As Rique mentioned, "best" singers is a very ambiguous phrase that is wide open to interpretation. And while Ella Fitzgerald and Sarah Vaughn are musical legends, I woud not recommend their recordings to someone who is hoping to learn English from the lyrics. From the most modern popular music to the songs made famous during World War I, the lyrics regularly contain slang, incorrect grammar and nonsensical words. Even if the lyrics are correct English, each singer's enunciation is different (Elton John is notorious for having his lyrics misunderstood just due to his pronunciation); I'm a native speaker and I still have difficulty understanding a lot of singers.