У тебя красивая задница
or У тебя красивый зад
Зад is more polite, задница is more blunt, (and the drect equivalent, tone-wise, of ass) although by no means offensive.
I think the chief consideration here is the atmosphere. If it's a given that someone, say, a man, is saying this to someone else, say, a woman, then there is no room for faux political correctness, dilly-dallying or euphemization of what is a very direct and straightforward compliment.
In this case, nothing but the real thing will do, and the real thing in Russian, as far as ass is concerned, is задница, not попа, not попка, not anything else, жопа excluded.
Красивая means beautiful.
I would say that, when taken together and pronounced - in the right situation! - with a certain amount of conviction and confidence, the adjective (красивая) and noun (задница) will fail to impress only the most prudish or self-abhorring member of the fairer sex.
Local time: 14:24
Native speaker of: Russian, English