This might have been the most wonderful moment in his life 此文法有錯嗎??為什麼是have been?

Don't ask "have been," but ask "might have", Or you use "may have" with a past participle when suggesting something happened or was true, giving possible explanation
Don't ask "have been", but ask "might have"-----
"might" is a modal verb used with the basic form of a verb.
You use "might have" with a past participle to indicate possible something happened,true,possibly explanation for something;eg:-I heard what "might have been" an explosion in Convention Center HK.....
Hence:-modal,pragmatic on "might have" ;-eg:-
On rearranging a sentence,we've:-
The most wonderful moment in his life "might have been":-
(1 ) getting married;
(2) promote to a high official post by the Emperor
(3) won a prize, have a windfall
(4) joining a party;
might have be--------with a present participle
might have been-----with a past participle
It is to indicate that it is possible that something happened or was true with a possible explanation.
Hence on rearranging the sentence, we've:-
have been--->might have been
eg:-This might have been ----->the most wonderful moment in his life.

This(主詞：第三人稱、單數) might(助動詞) [has(原形動詞：第三人稱、單數) been(過去分詞)](現在完成式) [the most wonderful moment(本句係不可數名詞：可數、不可數名詞因其釋義而定)](主詞補語) in his life(時間副詞).

From 劍橋字典
might-have-been
something positive that could have been achieved, but was not:
某正面的事情，在過去有機會被達成，卻錯失了機會沒有達成
This might have been the most wonderful moment in his life
他當時有機會得到他這一生中最好的一刻，可惜他沒有達成.
本句意思如上，文法當然沒錯 – 助動詞may, can, might, could… 後接動詞一律是不帶to的不定詞原型，本句為have.
在這句中might 為過去式 + have + PP ==> 代表過去有機會完成
叁考一下假設語氣文法，上句的疑問自然而解