Isaac is ineligible to be the least favourite anything just on the strength of his presentation alone. He's really brought to life by vivid facial expressions, as well as excellent body language and voice acting courtesy of Liam O'Brien. Unquestionably one of the best villains in the series. Also someone mentioned Rondo Maria, and to that I say bollocks. Bollocks, now and forever.
Uhhhhhh... everyone from the first two DS games spring to mind. Even if the concepts are salvageable like in, say, Brauner's case, the grim reality is that the inept dialogue and art style ruins any and all specks of mild quality in them. Skip, skip, skip. The cast of Lords is much the same. No one acts like a human being, and there is no logic to anything they do. I've never seen so many words used to convey so little. It's not visually exciting either; half the cast wears the boring Brotherhood get-up.
I have this unquenchable distaste for Joachim. I mean, sure, he's a glorified plot device but... guy's just really hateful to me. I'LL BEAT YOU... TO A PULP. It's bizarre, because he's also voiced by O'Brien. Oh well.