。
My opinion is completely different from yours.
Literal
I [of-の] opinion [topic-は] you [from-とは] completely differs.
君とは stacks the contrastive topic marker は onto と — 'from you specifically, as opposed to anyone else.' The contrast is nearly imperceptible in English translation, but native speakers feel the targeted emphasis. The bare 君と version reads as a neutral comparison; this とは version sharpens the focus on the addressee.