。
My opinion is the opposite of yours.
Literal
I [of-の] opinion [topic-は] your [one] [with-と] opposite is.
反対 ('opposite, opposition, against') as a na-adjective or noun. Here as a noun with the copula だ: X は Y と反対だ = 'X is the opposite of Y.' The standard-of-comparison と marks 'from' in this pattern as well. 君の elides 意見 — 'yours.'