What is the best living filter?

[BCOrchidGuy]

Okay, I've got a sump and not entirely pleased with it so I'm thinking about redesigning it but before I do I wanted to find out peoples opinions about what the best live filter is. Sponges are great but slow growing, what about soft corals? How about Macro Algaes? Gorgonians?
I'd like something I can keep a few blue legged hermit crabs with for bigger bits of crud but what would be a great water polisher?

Doug