I think Denth and Tonk Fah, while not necessarily being the scariest, deserve an honorable mention simply because they were able to gain Vivenna's trust so easily. That level of manipulation is pretty dang creepy.
I think I just have a thing for wholesome nerdy characters in general. Renarin doesn't get a ton of screentime, so pay attention to him, because I think he'll end up being the coolest character in the series.
Maybe the reason the artifabrians haven't been able to recreate shardplate is because no one thought to switch their calculators from degrees to radiants.