Favorite Story Arc
This one's tough...
I really like the Jeanne story arc, if only because you slowly realize she has NO IDEA who Tony is! LOL! I thought ,despite being a slasher, they were really cute together too.
BUT, The Ari arc was awesomely written. It had a great build-up to an explosive ending! So I'd probably have to go with that one.
One of the best written scenes of NCIS, in my opinion