I find this a difficult question to answer in regards to the regular actors, I find it difficult to distinguish between characters I'm not that fond of and the actor's performances.
Katherine Hepburn once told Cary Grant that he wasn't acting, he was just playing himself. Grant replied yes, but I play myself so well. We remember him for his performances not his acting. There is definately a difference between acting and a performance. I prefer Alexander Siddig's performance better than Nana Visitor's, but I don't think I could tell you who was the better actor.
As for the guest stars on the other hand, the female Vulcan maquis member from season 2 of DS9 was pretty awful.