It's been awhile but I have quite a few I don't like too much, Night Walker i'd say is the worst job i've ever played.
Shade is another bad one, pretty sub-par mobbing from what I was used to during the time (before 5th.)
Phantom I didn't like much either, i'd rather be playing the classes I stole the skills from, their main skill was cool I guess.
Kaiser I was excited for but very disappointed by, this was even before the nerf and it just was not fun at all to me, Blaster is just garbage if i'm going to be honest, I think everyone knows what I mean with the combos and stuff.
Mechanic was pretty bad when I played it (during chaos patch.)
I'd say my least is probably NW or Blaster.
As for the other classes, I liked the rest of them.