From what I've seen online, it seems like most of the classic Doctor Who
available for download is generally appropriately weighted towards some of the more well-regarded stories from the series: "The Aztecs," "The Tomb of the Cybermen," "Carnival of Monsters," "Pyramids of Mars," "The Caves of Androzani," etc.
I'm quite fond of the Key to Time arc. That's the story arc that first got me into the show. I love the banter between Tom Baker & Mary Tamm.