your top five (low five?) least favourite characters

By 'The second episode he appeared' I pray you mean when he inhabited Sisko. If you mean that Ezri 'Vulcan murder' fiasco, that episode is 'Worst of series' bad.

Right up from the first scene where they're being so nice to this random stranger you know minutes in advance he's about to die, you know the episode is going to be 'Hammer on head' cheesy. Then the rest of the episode plays out like the worst writer on the Criminal Minds staff wrote a Silence of the Lambs sequel. Not only is there a Vulcan who believes that killing people on the basis of seeing pictures that express emotion is 'The logical thing to do', Joran somehow figures out he's a Vulcan simply based on seeing these pictures.

Every shot, every line of dialog, and every slice of plot logic in that episode is facepalm-vomitrocious. He seems less like a cold blooded killer, and more like the bad guy on a bad crime procedural.