Sykar
Arcane
Archdemon, because it was the most uninpsired, trite, boring and idiotic character ever. The story never tells you why he attacks, you learn nothing of his motivations, etc. No one else is blander or more forgetable among Bioware's antagonist, albeit they have quite a few shitty ones.
Ultimately, you cannot even call it a "character" since it is closer to a lego brick Bioderp needed to fill that big gaping hole in their story with a big bad.
It's a big fucking diseased dragon, what sort of motivation does it need? The reason for it attacking is kinda the whole mystery of the series.
There is no ingame indication that the archdemon is driven insane or that it is driven on some other reason whatsoever. You get a few growls and that is it, it attacks "cuz reasonz". I am not even saying there would have been a need to make a fully detailed lengthy explanation, but come on, at least give us a hint, a glimpse, something.
Flemeth was an evil dragon as well but got a lot more background in comparison.