I've been using a cheap polyester t-shirt, short sleeves.
I haven't tried it yet, but I just got a button-up fishing shirt ($30, Field & Stream brand at Dick's Sporting Goods) which is long-sleeved, collar, stretchy polyester, and has a vented mesh panel in the back. Columbia makes a similar one, but this was slightly more comfortable in the store and cheaper. I also treated it with permethrin.
A nylon shirt won't wick and dry as well as polyester, but nylon is more durable.