Pertwee to Baker was the one that sticks in my mind from when I was a kid, as this was around the time I first got into Dr Who - so that would be my fave.

Colin Baker to McCoy was pretty pants really and it's a shame Colin B didn't come back for a proper regen scene.

In fact, most of them have been fairly dire. Jon Pertwee just falling out of the Tardis??

Have to agree here [hello - this is my first post by the way]
I think the Hartnell-Troughton was mind-blowingly simple, but shocking because it had never been done before.
Nowadays it's just accepted that there's another Doctor round the corner.

Davidson! Gotta love him. BURILLYENT!

Davison to Baker:

'cause

a: He gets to do some of his best acting of his era

and

B:We get to admire the twin magnificence of Nicola Bryant as she completely steals the scene!