The Dura Ball pro. I've seen a clip of it being used as a toy by a baby elephant! And Paul Check does weight training while STANDING on it (let's hope Cathe doesn't get any ideas!LOL!).
I also have a Core Secrets ball, which is softer and seems pretty tough, but I've only had it inflated for about a week! (The bigger size and softer texture makes some move easier, like pikes! I was wondering why those were SO HARD to do on my Duraball pro, which is very firm. Turns out, I was getting more of a stability challenge with it)