I don't know about best, since I've only seen around 300-350 episodes from the 1966, 1967, 1968, 1795, and 1840 sequences. I did enjoy the one that aired yesterday. Very surreal camera work.
For worst, I'd have to say the dream curse ones. I HOPE nothing gets as bad as they did. I did enjoy the one with Professor Stokes, and I liked how they finally dropped the regular format with Vicki and Barnabas.