In the past I've done a little research looking for a decent OC spray. On several forums, the consensus seems to be that, as John said below, Fox Lab OC is widely regarded. It's the "hottest" stuff out there at 5 million SHU (my ASP palm defender only has 2 million SHU, but I like it's size). You'll probably want to make sure you get stream or foam to minimize exposure to yourself when you're bouncing around trying to avoid the dog. Any sort of wind might send fog or cone right back atcha.