There was one great addition in the later years of the show - April. Sheree J.Wilson played April perfectly, saving seasons 11, 12 and 13 from being considerably worse.
James irritated me too. That's a bad character and a bad actor as opposed to Cally who was not a good character because it just didn't fit in Dallas, but Cathy Podewell as an actress did as well as you could reasonably expect anyone to do given that role. Cathy Podewell was convincing as that character, but unfortunately it was a character that was badly out of place on Dallas. Still, Cally had far more charm than Michelle or Rose or Leslie!
I say Leslie Stewart was the worst character because it was a bad concept for a character and terribly written, which really marred what was an otherwise amazing season. By seasons 11 or 12, there were enough bad characters that one more couldn't do a whole lot of damage; it could only take it from bad to worse. In comparison, Leslie Stewart was the worst because all the rest of season 4 was so good that it was a situation where her character badly tarnished what was otherwise a great season.
I compare it to a painting: if you have something great, maybe even a masterpiece, and it gets damaged, it's really tragic. If you have a painting that is just OK, and it gets damaged, it's not as sad.
@Swami, I think Leeann De La Vega and Peter Richards both could have fit on your list very well, but it's hard to remember everyone. What did you think of those two?