I nominate
Point Blank by CC Pybus for best design. This map is undeniably the best designed map on foundry I've seen. This map promotes what is lacking in almost all foundry maps today, RAMPS. You've seen it happen before, and you probably have had it happen to you also. Being killed while jumping to gain ground or a weapon is one of the more common deaths in Halo 3. ( It's a very irritating one too. )
This design drives the game play, it doesn't stop it with the use of stairs, angled bridges, and fence walls. The basement portion is absolutely amazing, definitely by far the best design I've seen yet on foundry. He uses the bridges as a form of ramp while also making it seem like a simplistic bunker, where teammates can spawn. As for the forging in general, one word: PERFECT. Link:
Point Blank