The Raramuri (as they call themselves) or Tarahumara (as the Spanish named them) are quite famous both as perhaps the least acculturated people in the Americas and as world-class runners. (Raramuri means something like "people who run".)

They're known to be quite reticent about picture taking and overall interaction with outsiders, and I didn't get to know any, but was fascinated to see them once I got to see many outside the contect of simple crafts sales and such. Seeing native peoples in their native costumes in a park in a tourist town is not too exciting, but backpacking along and coming to a town in the middle of nowhere and seeing that they actually wear the real traditional garb is pretty amazing.

I didn't take pictures of any Raramuri, but found some on the web that might be interesting to you. The men wear a skirt-like cover over a loincloth:

The women wear wonderful multicolored skirts and scarves and everything. I've never seen a more beatiful set of clothes on anybody anywhere than the bright colored getup that a woman had on the bus from Batopilas. It was simply gorgeous, with so many beautiful colors.  

The Coppercanyon-Mexico.com site has a bit more on the Raramuri.