Aza Industries was pretty good at elaborate structures, like castles and church-like buildings (I say that because one of them reminded me of one a lot, and another seemed based off of one). He used to build them all the time on Badspot's server, at least, when it was a terrain build.
Barnabas is also a really good builder, I've seen a few of his builds on Badspot's server and on his own. They were pretty good, at least, in my opinion.
Ladezkik and Eksi/Squideey are both all good builders. I've seen several builds by Ladezkik, and they were pretty good. I've also seen a house on Eksi's server, I don't know how much was by Eksi or Squideey (I know it was by both of them), but it was still really good.
On a side note, I've built pretty good houses c:
EDIT: Off topic, but does anyone remember that jet ride thing Ephialtes built on Badspot's server? That was fun :D
