Which bacteria have the highest mutation rate?
You seem to be assuming that mutation rates are somehow constant over evolutionary time. They are not. Mutation rates will change according to the stresses a species is subjected to. If you take a bacterial population and place it in a stressful environment (high/low temperatures, oxidative stress, lack of nourishment or whatever) you are likely to see an increase in the mutation rate.

That said, this review and references therein might interest you:

E. Denamur and I. Matic,Evolution of mutation rates in bacteria,Molecular Microbiology 60(4), pages 820–827, May 2006