I have sensitive skin, perspire a lot & live in a very humid, subtropical climate - all the more reason to stock up on very breathable & machine washable clothing, that can really take a beating.
Are Rayon & viscose the same fabric? I ask because I have some 100% viscose, printed long shirt dresses from Zara that fit the above bill perfectly (they wear like iron yet feel both cool & drapey even in 30c+ weather. Yes there may be a few wrinkles, but they disappear if you immediately hang them to dry & the prints are fairly forgiving). This may have skewed my perception of viscose a little, since I've bought these dresses in different years & yet found the quality to be consistent (I haven't experienced the fading you mentioned, even in the black dresses).
That said, I've had horrible experiences with garments labelled as 'rayon' in the past. If they are indeed the same material though, then I may need to rethink my 'fabric first' approach to shopping for new clothes (e.g. I'm totally with cindysmith re: polyester - both that & acrylic are absolute deal breakers for me when shopping).