First, the perfect ass needs the perfect life support system surrounding it. A slim, firm, but curvy dancer's body, and non-working vocal cords. Ha ha! Just kidding about the vocal cords! No I'm not!
Okay, the ass: perfect heart shape, moderate projection (although on a slim body, moderate projection is still impressive).
Asshole: It should be reasonably close in pigment to the skin surrounding it. Not much much darker, and not shockingly more pink (which is why I don't love redhead's assholes, usually). Obviously, I like an asshole that does not have skin tags from previous hemmorhoids, but for the most part, this is only an option on younger women.
Smell: obviously, the less the better. I know what you're thinking: this is strictly a matter of hygiene. As a qualified medical professional, I chuckle in derision at what a moron you are. Obviously, you can't get past lack of hygiene, but even with great hygiene, if she's unlucky enough to have a sphincter that lets through even a little fluid, it'll smell. I had a gf whose asshole I could only lick in the shower ... She was perfect fresh immediately afterwards, but even an hour later, she was getting a little funky. Another girl I dated, had no detectable smell 5 hours after she showered. That was all just genetic luck, they were both clean women