Castle because I just adore Nathan Fillion. Then I would have to say Martha because I just love her classy style and her quirkiness. It's really hard to choose a favorite though because I love them all.
Castle's daughter, Alexis. One, because we share the same name. Two, because her and castle have a very close bond that is evident when they are together. Alexis even unknowingly helps castle solve cases with beckett.