The belle of the ball
The most attractive woman at a social gathering.
This phrase is, of course, 'the belle of the ball', not 'the bell of the ball', as it is sometimes incorrectly spelled.
A ball is of course a social gathering for dancing etc. 'Belle', meaning 'fair; beautiful', was adopted into English in the 17th century from the Old French 'bele' and ultimately from the Latin 'bella'. A specific meaning, the one used in 'belle of the ball', was 'a lady who is the reigning beauty of a place'. This was known by at least 1622 when John Fletcher used it in his comic play The Beggars Bush:
"Vandunke's daughter, The dainty black-ey'd belle."
'Belle' has been, and continues to be, used in various French adjectival phrases, for example, 'belle assemblée' - brilliant assembly; 'belle dame' - fair lady; 'belle laide' - an attractively ugly woman; 'belle passion' - tender passion. The term 'belle of the ball' is of English origin. At least, the earliest citation of it that I can find is from an English journal, The New Monthly Magazine, 1822:
"The partner of partners, the belle of the ball."