30 Days of Dr Who - Day 29:
Favorite Time Lord that's not The Doctor....Doctor Donna! Presuming she counts, which I think she does. True she couldn't regenerate, but she had all that knowledge, etc, that a Time Lord/The Doctor would.
(Confession, it's hard to pick just one! I LOVE River Song as well - she counts given her initial ability to regenerate and such in my opinion - and, of course, the madman that is The Master.)