。
This is my strongest area.
Literal
This [subj-が] most skilled field [copula-です].
得意な分野 'area of expertise / strong suit' uses 得意 'skilled/good at' as a na-adjective modifying 分野 'field.' 一番 'number one / most' is the standard superlative adverb. A compact, textbook-level self-description sentence.