I'm assuming we're talking favorite big bad's, not favorites in general? :-P
Spike and Dru were my favorites. Or Dru if you don't wanna count Spike as he became good, but I loved him and Dru in the second season when they were the big bad so yeah.
But I agree Harry was great!
Really, Faith's journey in that season is just a very intelligent case study on the psychology of it all.
Worst Big Bad...you really need to guess that it's Adam?
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli