"The Aztecs" is one of the best early ones, and it's interesting to see again. It's got some good drama and humor and delves into some interesting ideas. And I gather from the DW Wiki that its portrayal of the Aztecs was pretty well-researched, though there's still a certain cultural condescension -- and the pronunciations of Aztec names are all very wrong. Of course one has to be able to look past the crude video, stagey production values, and stilted performances, but that's just the nature of '60s BBC-TV.
It's particularly impressive to see how strong a character Barbara could be, how commanding and intelligent. We didn't see another female lead like that on the show for years after she left. The ones that followed -- Vicki, Dodo, Polly, Victoria -- were all pretty much just cute young damsels in distress who screamed a lot. The next one after that, Zoe, was a genius who could be quite resourceful, but still young and vulnerable. We didn't really get another female companion on a par with Barbara until the Third Doctor's era and after.