Is this a set phrase: "best of all"?

[Dima]

Test No. incompl/elem-15 "An Easy Question", question 1

What exercise do you like doing ......... of all?

(a) next
(b) best
(c) after
(d) before

Test No. incompl/elem-15 "An Easy Question", answer 1

What exercise do you like doing best of all?

Correct answer: (b) best

Your answer was: incorrect
What exercise do you like doing before of all?
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why is best ? I chosen 'before'?

Dima

[Alan]

You cannot say before of all - two prepositions here do not make sense. You need best because this is the superlative form meaning the most favourite.
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English as a Second Language
You can read my ESL story Phrasal Verbs/run

[MadokaLiu]

[Manjula]

why I can not use after all.