My vote lies against metalworks and for Warmfront
Although Yukon would be my second choice.
I didn't like metalworks because of its weird chokey architecture from mid to 2, and all the twisty turney rooms headed into last. I see what was being attempted - a valley like in blands = flank, but unfortunately that area is simply too big.
Warmfront is a fine map, and although last can be rough, it's not impossible by any stretch of the imagination. Yukon is a little harder because of height problems and the pipe thingey + mid fights are really weird.