Slazenger Wimbledon Ultra-Vis is about the best playing ball you are ever going to get. Seems like an expensive indulgence for a ball machine. But hey, if you can afford it!
For a ball machine, I don't think its matter so much because you're just doing drills. It's important that they've got a reasonable degree of bounce, reasonably fresh and economical to buy given how often you'll need to replace them.