TRICKSY! My first thought was to post my favorite line from New Who, but I think this time I'll go Classic.
"A man is the sum of his memories, you know - a Time Lord even more so."
The Fifth Doctor, The Five Doctors
It's just too difficult to select just one and trying not to repeat what I've already seen. A lot of good quotes in "The Five Doctors" too, especially the one from the Master that everyone knows and loves.