What is nicest ass?

It seems like you might be referring to something specific or using colloquial language. If you're talking about a physical trait or a specific person or celebrity known for their physique, it's a subjective topic, and what one person considers "nice" might vary greatly from another's perspective.

If you mean something different, could you please provide more context or clarify your request? That way, I can give you a more accurate and meaningful response.