Which bacteria have the highest mutation rate? From my reading on _M. tuberculosis_ , I know that this organism has a pretty high mutation rate due to uncorrected sloppy replication, which leads to a high rate of development of spontaneous resistance to treatment. Are there species that can beat _M. tuberculosis_ in this degree of mutation, due to polymerase sloppiness or other mechanism?

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