Cotton! I ride in cotton t-shirts with the arms cut off and regular cotton underpants under nylon running shorts and cotton socks.
Hang them over your bike or tent overnight they'll be fresh as newly washed by the morning. 5 days between washes no problem.
All you guys buying $100 smelly synthetic t-shirts that bacteria and funguses thrive in, crack me up. The sports industry has got you bent over!
My t-shirts cost like $5.
Flame away!!! 
Actually, one can pick up synthetic shirts for $5 at some clothing liquidation stores around town here. Soccer jerseys are brightly colored, but I also like nice loud and ugly Hawaiian rayon shirts in warm weather. I can pick up nice synthetic athletic socks at the dollar store, but since I'm usually riding with no socks in Shimano SPD sandals, I really look like the "ugly American". I draw the line at plaid Bermuda shorts, though.
I also know of a cycling wear manufacturer here in Santee, CA, where I can pick up nice shorts and jerseys at around 1/4 to 1/3 the price in shops, for my Sierra touring, about 1.5 mile south of the ACA southern tier route, but it's mine...my precious... OK, a hint, it's very near the Chemtronics duck pond.