I can't really answer that, obviously the Amuse is highly praised and is Titanium but I'm not sure if its even available any more and who's going to pay £3k for an exhaust on a 350z ?
The Motordyne or Fast Intentions are widely regarded as the best, if I still had a Zed I would probably buy a Motordyne. Both these are built to order, a lot of others are mass produced, make of that what you will, it's not necessarily a bad thing
Just out of interest, what would you suggest is the nailed on "best" off the shelf system? I'm thinking gains, not volume.