the earlier the better for me! Since "This Is Janis Joplin" isnt a regular release, I would say the best is her first with BBHC - the Mainstream Records release Big Brother and the Holding Company. Lots of folk will say Cheap Thrills or others, but for me this one has the great sound!
The "Kozmic Blues" album would probably be the most depressing, especially the three last tracks, "Kozmic Blues", "Little Girl Blue" and "Work Me Lord".
agreed with mr. samson... to bad there arent extensive live recordings of big bro available for free download