I'd be inclined to nominate Einstein as the most intelligent non-living person, and Stephen Hawking as the most intelligent person living, but measuring intelligence is very difficult - accademic intelligence is not eh only measurable form, and IQ Tests only tend to prove that you can do IQ Tests, so it does come down to opinion, and I think most of the people who think about it would probably offer the same names as i have - but you may find differences here on the site.
Issac Newton once said 'If I have seen further, it's by standing on the shoulders of giants.' (May not be the exact quote). Obviously both Einstein and Hawking are phenomenal intelects, but I think as the most intelligent person to have lived, I'm inclined to say the Greek philosopher Plato, since he kick started many of the key ideas we're still building on today.
To paraphrase a wise man, "Wisdom is knowing that we know nothing."
The greatest minds think laterally and have the ability to look at things from a different angle to others. They have wild imaginations, marvellous insight, the ability to think for themselves and the courage / confidence to challenge conventional theories, ideas and standards / moral values despite possible outrage and vilification. I think I've said enough.
Intelligence is a matter of opinion. There's no definitive answer to your question.