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.