Lauren Bacall apparently started out with a high nasal speaking voice (and a Brooklyn accent), Richard Burton with a light voice, Kenneth Branagh with his native Irish brogue. Michael Caine, however, didn't alter his Cockney speech but cashed in on it. As Sean Connery did with his Scottish.
As for the "perfect" male English-speaking voice, I've read that that would be a combination of Jeremy Irons and the late Alan Rickman. With Judi Dench being among those with the "perfect" female voice. Then there are those actors (such as Rupert Penry-Jones, Samuel West, and Jeremy Irons) who also do narration. Really listen to them sometime. Each syllable is given its due. Their speech is slow, precise, crisp, beautifully articulated. No slurring rush about any of it. It's a pleasure to listen to. The tone color and resonance of a voice is also highly important. Sorry to say, I hear a lot of too-high, little girl voices by women (Americans, often) who, to my mind, might do well to re-see Singing in the Rain--which was all about using a well-modulated voice (Debbie Reynolds's) to dub over the story-line's leading lady's laughable pitch and pronunciation.
So (getting back to the subject) what did Lauren, Richard, and Kenneth do to deepen their voices or change their regional speech? Or, what did their directors or voice coaches have them do? For two--Lauren and Richard--the hills became alive with the sound of Shakespeare as they shouted out verses (it is said) for "hours at a time." Whereas Kenneth turned to something called Received Pronunciation. Or RP for short. He was born Northern Irish but, at age 9, moved with his family to England where he is said to have acquired RP in order to stop being bullied about his accent.
So what is Received Pronunciation? It's something like a BBC-tinted English-language pronunciation based on educated speech reflecting an upper middle-class status. It is clear and precise. Short vowels, not drawn-out drawls. Enunciation. Taking time to speak, not rushing it. It is not "a royal accent" as the royals are said to have their own way of speaking. It is thought of as neutral, not reflecting the speaker's geographical origins. Anthony Hopkins, for instance, is a Welshman (as was Burton) but did not play Lear with a Welsh accent. He and other Shakespearean actors and actresses--unless, say, they were playing Dogberry in Much Ado About Nothing--used RP or something akin. Dame Eileen Aitkens, who spoke with a Cockney accent as a child, switched to RP when beginning her work in the theater. I read somewhere that she said that one's native regional accent would not do for the theater's great roles.
As for RP, I guess one could conclude by calling it something of an Oxbridge style. It is also said to be Bond's accent, the 007 guy.
Getting back to the most pleasing English-speaking male voices, those would include Ronald Colman, Jack Hawkins, Pierce Brosnan, Burton, Hopkins, Irons, and Rickman. Female speaking voices would include the great ladies: Julie, Maggie, Judi, and Helen.
Finally, speaking of Alan Rickman, listen to his reading of Shakespeare's Sonnet 130. It's amazingly beautiful. (If for some reason it does not come up here, Google it.)