何より・何といっても: the difference
何よりN2
Meaning
Be glad, Be relieved, Most, Best, Above all else, More than anything
Past papers
Example
「ありがとうございます。」 「お役に立てれば、何よりの幸せです。」
'Thank you very much.' 'Nothing would make me happier than being of help.'
何といってもN2
Meaning
Above all, After all, Whatever anyone says
Past papers
Example
彼の長所は、何といっても朗らかなところだ。
His strong point is, by any account, his cheerfulness.
How to tell them apart
「何より」: the most precious and welcome thing in feeling and value (health matters most, safety is best).
「何といっても」: 'whatever you say', the most decisive point is...: 「何といっても彼の実力が一番だ」 (Whatever you say, his ability is the best).