The original 26-year run concludes with the ironically-titled Survival. (Gloria Gaynor can relate, I'm sure.)
It lacks the deeper themes and clever storytelling of the two preceding serials, and, due to the presence of Cheetah People, is far, far sillier. But it's a fun, if nonsensical, straight-up Doctor versus Master story, and is significant both for the last appearance of Ainley as the Master (a 1990's adventure game notwithstanding) and of course the series finale. Plus it explains what the Master's doing with yellow cat eyes in the 1996 TV movie.
$25 at Amazon; comes with a second disc that apparently has a lot of extra features dealing with the historical significance of the ep. It's not available streaming on Amazon Prime or Netflix; if you're still doing the disc version of Netflix I'd say get it that way, and the second disc is optional. The serial's worth checking out, but its predecessors are better.