What bacteria wins the prize re ability to withstand temperature extremes, chemical assaults etc.? What bacteria wins the Toughest Unicellular Microorganism competition?
Deinococcus radiodurans would have to be in the running…
Clearly, this depends on your method of extermination.
As far as common human pathogens, the botulism spore is the standard. Clostridium botulinum forms incredibly tough spores when it’s stressed. They’re used all over to test if sterilization procedures are working properly - you put a vial of them through whatever process you’re testing and see if they can grow. If it kills the spores, it’ll kill anything else.
Bacteria are notoriously differentiated in their survival ability. As beowulff mentioned above, Deinococcus radiodurans is the most radiation-resistant bacterium. This microbe and its close relatives survive immense doses of x-rays and gamma-rays.
This needs to be repeated, often. It depends entirely and totally on your method of extermination. Those bacteria resisatnt to radiation aren’t very resistant to bleach. Those bacteria that can survive high bleach concentrations don’t handle high temperatures, those that can withstand high temepratures are UV sensitive, those that are UV resistant don’t handle dessication and so on and so forth.
There simply isn’t one most retsistant species. There is a separate and distinct species that are most resistant to each and every bactericidal treatment you can think of.