This is "What's your favorite Star Trek series?" by another name...
... but my answer is Kirk.
Now that's interesting. Are our favourite series generally defined by the captain or lead? Would our rankings in leads and series be pretty much identical? With the exception of DS9 and Sisko for me (I like the show more than I like him since I really don't care for him as much as the surrounding cast), it's probably mostly true.
I cast my vote for Picard. TNG is also my top spot.