The Orbee-Tuff ball on a string is the best I've seen. I cannot tell you how amazed I was when I first picked it up. A friend of mine who is a professional dog trainer told me about this ball. Her personal dog has had an Orbee-Tuff ball on the string for two years and LOVES IT!
One of my most popular training DVDs is Building Drive and Focus. The work in this DVD involves building prey drive through drive development using a ball on a string. In that video I go into detail on why it's so important to use the right kind of ball for this training. No matter how good you are at the work you are going to mistakenly hit your dog in the head while making drive with the ball on a string. This is normal and no one is perfect. The problem is that when you use a hard rubber ball and hit your dog in the head - rather than put drive into the dog - you end up hurting him and this results in taking drive out of the dog.