I would have to choose Abbey due to her always up-beat personality and because she's the happiest goth you'll ever hope to meet :) She's always there for the team, whether it be professional or personal, and I love the sweet father-daughter relationship she shares with Gibbs.
Ziva comes a VERY close second. I really admire her mix of both vulnerability and ruthlessness. She learns a lot about herself after coming to NCIS, and finds genuine people who really care for her and who become her un-official family. One of my favourite Ziva moments is when she tells Gibbs he is the closest thing she has to a father.
Jenny is probably the closest Gibbs ever got to a second chance at true love after Shannon died, and I love how she is still in love with him, and her spunky personality.
Kate was no doubt a valuble asset to the team, but her time passed, and it became Ziva's chance to learn what she could from the 'coffie afficianado and functional mute' with the list of rules to govern both work and life.