〜ことこの上ない
nothing could be more, extremely
い-adj / な-adj + な + ことこの上ない
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ことこの上ない means "nothing is above this", so "as … as it could possibly be": 山の上から見る夕日は美しいことこの上ない, "the sunset from the mountaintop is as beautiful as it gets".
It follows an adjective: い-adjectives directly (美しい, 嬉しい), and な-adjectives with な (危険な, 退屈な). It works for positive and negative feelings.
It's formal and written, and it sounds more measured than 極まりない. You can also use この上ない as an adjective before a noun: この上ない喜び, "the greatest joy".
Don't confuse it with ことはない (N3), "there's no need to".
Examples
彼の説明は分かりにくいことこの上ない。
His explanation couldn't be harder to follow.
山の上から見る夕日は美しいことこの上ない。
The sunset from the mountaintop is as beautiful as it gets.
夜道を一人で歩くのは危険なことこの上ない。
Walking alone at night couldn't be more dangerous.
毎日同じ作業ばかりで、退屈なことこの上ない。
Doing the same work every day is as boring as it gets.
優勝できて、嬉しいことこの上ない。
I couldn't be happier that we won.