The one in the middle that uses Swiss mags is probably my favorite as far as collecting. From the functional perspective, it's definitely the 556xi on the left. The SBR on top is probably the most fun to shoot (but not so for any bystanders).
While that'd be cool, it would be too much $$. Without even getting into the Swiss SAN stuff, I've already seen 5 different styles of US-made lower receivers in just 5.56 caliber - and that doesn't even take into account the different permutations of those lowers - some are hinged, some are AR-style. I had the SIG 556 bug for a long while and just collected them as I had the disposable funds and would find one at the right price.