Me and a friend of mine got in this argument tonight. Mine personal favorite is Data, by far I think hes the best of all time. Even though he an android I feel he is the most human character on the show. My best example would be the episode where Data tests the saying "a watched pot never boils". I like this alot because hes a robot and he measures time exactly, but when Riker tells him to turn his internal timer off he notices the pot boil "sooner" than average. I think this displays an uncommon variety of humanity amongst a group of humans who seem to know everything there is to know about the universe. Data, for the sake of knowledge, discounts all the factual inputs he has for such an experience in order to learn from it, that's why I think hes probably the most human character on the show because he discounts factual knowledge for pure experience aside from fact.
The Brain Trust