Weird, I always considered season 6 a weaker season than 3-5, but I'm finding it hard to knock an episode off. These are all decent episodes.
I vote Chain of Command (I). I haven't seen this in a while, but it felt like the whole episode was Riker bitching about Jellico, and Picard/Crusher/Worf fumbling through caves. Part 2 was a much better episode.
Realm of Fear
Chain of Command (II)
Face of the Enemy
Frame of Mind