I also have another request for title recommendations. I've pretty much picked out which remaining Tom Baker stories I'll get this time around, but I'm having trouble with the next three Doctors. Part of the problem lies in the relatively slim pickings for the final two. These are the ones out on DVD.
Peter Davison ("Castrovalva" is part of the New Beginnings box, which I'm certainly going to buy)
The Visitation
Earthshock
Time-Flight/Arc of Infinity
The Five Doctors
Warriors of The Deep
Resurrection of The Daleks
The Caves of Androzani
Colin Baker
Vengeance On Varos
The Mark of The Rani
The Two Doctors
Timelash
Revelation of The Daleks
Sylvester McCoy
Remembrance of The Daleks
Ghost Light
The Curse of Fenric
Survival (Kind of already set on getting this, since it's the last of the original show)
You will definitely want Castrovalva, since that continues the story of the Doctor's regeneration in Logopolis -- and takes up about 10 seconds from where the previous story leaves off. Castrovalva is also the continuation of a loose arc of stories involving the Master, and the beginning of Nyssa's odd habit of permanently losing track of her clothing. Oh, and we find yet
another use for the sonic screwdriver (please, don't anybody spoil it for S. Gomez, as it's worth a giggle!).
Of the rest of the Peter Davison stories, those I would consider essential are:
Earthshock (a Companion leaves and Tegan utters her famous self-descriptor: "I'm just a mouth on legs!")
The Five Doctors -- includes Companions from waaay back in the '60s, and nicely mixes up the Doctors and Companions. Also, there is a hilarious Doctor Who/Survivor spoof on the 'Net that uses elements of this story. The spoof isn't canon Who by any means, just a really fun read.
Basically, any story that deals with a Companion arriving/leaving, and the Doctor's regeneration -- they're all powerful stories. And I would also suggest Enlightenment, as that's an important part of Turlough's character development.
The Colin Baker stories... well, the regeneration ones, of course, although he never shot one to regenerate into Sylvester McCoy. The only one I consider a "must-see" is The Two Doctors, because of Patrick Troughton and Frazier Hines (as Jamie McCrimmon -- still young-looking and sexy after all this time!

).
Sylvester McCoy's stories... Unpopular opinion, here, but I LOVED Paradise Towers. If you like "Britcoms", there are two terrific actors in this one -- Richard Briers from "Good Neighbors" and the woman who played Daisy in "Keeping Up Appearances."
My favorite McCoy story is Battlefield, because of the return of Brigadier Lethbridge-Stewart, the introduction of Brigadier Winifred Bambera (she would have made a wonderful Companion!), and the way UNIT is
finally shown to be a truly multinational organization. Really, this is a definite must-see story.