In general, just having a balanced amount of muscle definition and fat makes your butt nice. I knew a girl who worked out a lot and her butt looked AMAZING in jeans but it was unnaturally firm and felt kinda funny. I also know girls whose butts look AMAZING in jeans and they don't work out at all and their butts are kinda saggy once the pants are off.
Really it comes down to having a butt that has a bit of muscle on it but a bit of fat also. Women store fat on the butt more so if there's no fat there, your butt tends to look a bit too masculine. When the clothes are off, a nice butt really will just hold it's roundness in place so it looks very aethetically pleasing. Having a nice curve and a lack of sagginess is good. It's pretty much the same thing that makes guys' butts look nice, but girls' butts should be a bit more plump and a bit less muscular.
Imo, you've got a nice butt pretty much by default if you're fairly athletic. Just being in good shape for your gender makes your butt nice right there.
As far as celebrities go, Jessica Biel's butt is one of the nicer ones.
link link link Of course, your butt does not need to be this good to get a guy's mind running. You see butts looking a lot like hers every day honestly, but the fact that hers is a TINY bit nicer than all those makes it a highlight. The difference between a good butt and the "one of the best" butts is actually pretty small and both will do pretty much the same thing to my mood.
And they can come in tons of sizes. Jessica Alba (
link link ) and Kim Kardashian (
link link ) have equally nice butts, imo. It's the shape that matters most.
Hope that helps! If you have more questions I can talk about butts all day hahahaha.