Breasts are amazing anyhow, but anything between A and C usually does the trick with C's being the favorite. Keep in mind though bigger can work for bigger girls, but it's all about proportion Basic geometry
For playtime, don't care as long as they are pert. Often find more than a mouthful is a waste. My most sexually rewarding relationship was with a lady blessed with a nice round tush but "modest" A cups, and they were great fun, super-sensitive hotspots. She was tiny so in proportion they looked a lot bigger, and she looked amazing with her top off. They were the perkiest things ever. Plus with a push-up bra on and wrapped in a tight LBD she still pulled off "hourglass" for a petite lass.
For staring at in a state of masturbatory lust, I used to think C was my favourite, but having checked the stats of the pornstars that I think of first when I think "her tits look perfect" they are mostly D cups and the occasional DD. Although the older I get the more into asses I get, and really the "sexy" is more about the total package.