I'd say that Darth Bane was definately the best Sith (or atleast close to it). He brought the Sith order to it's peak and comanded a great amount of power.
Choosing the "worst Sith" is a bit difficult. Everyone (Sith or Jedi) has had moments where they've shined and moments where they haven't.
I wouldn't concider him the worst Sith in any way but I feel compelled to point out Darth Sidious (gonna get bashed for this I bet lol). I don't agree with alot of his policies. While his manipulations and power of command are spectacular, he did destroy the Sith order.
It started when he violated the Rule of Two and killed Darth Plageus in his sleep. If I remember correctly, a duel to the death determines whether or not the apprentice is ready to take the role of master. Killing him in his sleep seemed like the cowardly thing to do.
There's also the immense number of apprentices he had. I personally believe that he was too fickle with apprenticeship. She had, what, six or so apprentices? He didn't really spend enough time with any of them and just used them too expendably. This is what led him to die and take the Sith order with him (when he turned on Vader and didn't expect the retalliation).
Though despite this I wouldn't concider him the worst in any way. I was just pointing out a few flaws I spotted.
I also have some complaints about Darth Tyrannus and Darth Maul.
Though by default I'd say Darth Caedus was the worst (or atleast my least favorite). I found him both the most annoying and the most infurriating.
Edited on 01/25/2010 3:27pm