Jed September (responsible for that lovely machine to the right):
Jed has some wonderfully solid builds and very clean composition.
Soren is the official designer for MFZ, and for good reason. Everything he puts together blows my mind in one way or another.
Some very nice Commisar based frames and very clean color shcemes. This one is my personal favorite.
A really cool style, and how can you turn down a mech with a chair for a chest?
Not much MFZ related yet, but everything there is awesome!
There's plenty more around, and this list is by no means complete. There are a ton of other great builders and I encourage you to take some time and look through them!