I'll put my vote in for an ExOfficio Buzz Off shirt or something similar. I got mine about four years ago and really like it. It's similar to many of the Columbia shirts. It has long sleeves that roll up with buttoned straps to keep them up. The pockets zip shut (Columbia has velcro) with lots of room for flyboxes, leaders, etc.
I used to hike in a T-Shirt but this works just as well and has the added advantage of being a long sleeved shirt for a bit of extra warmth/sun protection or the extra long sleeve layer later in the day. Mine originally was bug repellent, but you can get permethrin spray to apply to your clothing. It's made out of cotton/polyester and dries very quickly. I wash it during the hot part of the day and it dries almost immediately. I got mine on sale from National Geographic, but you can probably find them on sale every now and then.