early planing, lighter weight, maneuverable, relatively durable
or did you mean "who" makes a good freestyle sail? 
Generally - Bit of a flatter shape, light in the hands and not to powerfull so a high wind wavesail or freestyle sail. and of course the smaller the better.
I have a 5.8 poison that I use in light wind which is a powerful wave sail but it is also much more work to freestyle with. Love it when I finally get to use my manics because the weight and power difference is noticeable.
Good question. The answers below are the objectives of a freestyle sail, but its characteristics to achieve those objective aren't clear to me, and freestyle is all I do (try, actually). Nowadays anyways all sails are lighter than ever.
I just renewed my old freestyle stock, from 5+6m on an old Exocet to brand new 5.2+6m (Aerotech) on a Goya. Yes I like the power, but I recommend underpowered to anyone else freestyling.
Before I bought, I spend countless hours researching, as my knowledge was way outta whack. Did all the vendors sites and forum, etc. I have found the sailmakers sites confusing about this issue. To most of them, there seems to be little diff between wave freestyle and flatwater freestyle, and even plain wavesailing.
All I know I'm having a hard time getting used to the new stock, I don't like the speed at which my new sails depower above my head, but that must be me and more adjustments being needed...
Good luck on your purchase.
