〜にもまして
even more than, above all
Noun + にもまして · Question word + にもまして
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にもまして means "even more than X": 今年は去年にもまして暑い, "this year is even hotter than last year".
The point is that X was already remarkable, and now it's exceeded. That's the difference from plain より, which is just a comparison.
With a question word, it means "above all": 何にもまして健康が大切だ, "health matters more than anything", and 誰にもまして, "more than anyone". It's also common with time: 以前にもまして, "even more than before".
It's formal and written. The ました here isn't polite ます. It comes from 増す, "to increase". Don't confuse it with にしても, "even if".
Examples
今年は去年にもまして暑い。
This year is even hotter than last year.
何にもまして、健康が大切だ。
Above all, health is what matters.
以前にもまして、仕事が忙しくなった。
Work has got even busier than before.
優勝も嬉しかったが、それにもまして仲間の言葉が嬉しかった。
Winning was great, but my teammates' words made me even happier.
彼は前にもまして熱心に練習するようになった。
He's started practising even harder than before.