Strictly speaking, this is more like Three-and-a-half Doctors and a Bloke in a Wig. Tom Baker didnt want to be in the story, which was made for the 20th anniversary of the show, and thus appeared only in clips from an unaired story. William Hartnell was already dead, so the role of the First Doctor went to Richard Hurndall, who looked and sounded quite a lot like the First Doctor, if youd never seen nor heard the First Doctor.
Also strictly speaking, this one doesnt really belong on this list. Frankly, its not a great story. But on the other hand, it is really wonderful seeing all the Doctors and various companions back again, and seeing the Cybermen, the Daleks, the Yeti, the Master and plenty of Time Lord insanity really does serve to make this something special, even if it isnt as good as some people might have wanted it to be.