Board Thread:Warhammer 40k General Discussion/@comment-66.206.224.153-20131222032846/@comment-18016240-20140226211236

For Orks I'd say Salamanders, fire dragons, or some other chapter that likes to use flamers and other weapons that not only would kill several orks per shot, but would also destroy the spores they produce, cutting down their population.

Chaos Space marines are more tricky, with Word Bearers in particular being annoying thanks to their close relationship with daemons. The Ultramarines and their succesors would probably be the worst ones to send up against them thanks to their rigidity with the codex making them predictable, and when dealing with Chaos one should always be able to think on their feet. I'd go with the Exorcists or Black Templars if we're not using the Grey Knights, since the Exorcists have firsthand experience dealing with daemons and their possessions and the Black Templars are zealotous even by Imperium standards.