My favorite multi-Doctor episode is “The Three Doctors” from 1972. The story featured the first three Doctors banding together in order to stop Omega, a renegade time Lord, from taking revenge on the other Time Lords for their abandonment of him. The serial is bittersweet because it is the last acting performance of William Hartnell, the First Doctor. He was very ill during filming, so he could only appear as a video screen image. Hartnell diedsortly after the serial aired.
The banter between the three Doctors makes the episode for me. The first Doctor is unimpressed with the dandy the clown (Second Doctor) ad dandy (Third Doctor) who succeeded him. The Second and Third Doctors try to one up one another while taking personal jabs. Patrick Troughton and Jon Pertwee enjoyed it so much, they kept up the banter in personal appearances long after quitting Dotor Who.
There are rumors Omega will be revealed in 2011 as the mastermind behind the TARDIS explosion that was the running plot thread of the last season. Rumors of returning villains float about all the time without ever materializing, so do not hold your breath. It would be neat to see him updated with better production vales than they have managed in the past.
