If your dogs like to play fetch or otherwise play with or chew on a ball, a tennis ball might seem like an obvious choice. The problem is that the fuzz on the ball acts like a bench grinder and wears down their teeth. Depending on the size of your dog, there is also the potential for a dog to get a tennis ball stuck in his throat.
A better choice is a ball without the fuzz and with good bounce potential. For larger dogs, we go with balls that are larger than a regular sized tennis ball.
We use the Chuckit! Ultra Ball, which lasts and is a great toy for playing fetch.