？
What's your favorite topping for sushi?
Literal
Honorific-sushi [in-で], number-one favorite topping [topic-は] what [copula-です][question-か]?
ネタ is in-group sushi terminology for the topping on a piece of nigiri or in a roll — derived from general 'material/subject' and reversed from タネ 'seed/source' as jargon. The お prefix on 寿司 is softening and polite. 一番好きな is an attributive adjective phrase 'favorite (= number-one liked),' with 一番 quantifying the degree of liking — the standard superlative construction.