I recently started going through the Third Doctor's seasons (having already seen his last season because that's where Sarah Jane joined). And while I was thinking about it, I came to a startling--and slighly heretical--conclusion.
John Pertwee's regeneration is sadder than David Tennent's.
Before you throw me into the nearest black hole, watch it yourself.
As heartbreaking as Tennant's is, it is obviously staged for all the dramatic value possible. The Oodsong, the explosions and fire, his prolonged delay...his last words. Pertwee's regeneration is smaller, more intimate, with Sarah Jane and the Brig really thinking he is dead....sniff...