I'll add a +1 for wool as the best of all worlds. Some weather conditions -- very hot and very humid -- are more comfortable in some synthetics, but in my experience, the smell factor overrides the extra comfort.
Thin wool -- 140/150g fabrics -- does very well. I've spent time in Texas and Hawaii, among other places, and done well in this in a broad range of temps.
In a perfect world, I'd recommend the Icebreaker Polos -- Detour Polo is my favorite. Weighs 6.3oz in a medium, looks great out on the town and is comfy hiking in almost any temps. Expensive, though. I justified getting a pair of Icebreaker polos before my honeymoon to Hawaii last fall. Figured it made packing easy... two shirts that I wore 90% of the time there, and I could go from an 8-mile day hike to dinner at a fancy restaurant without changing shirts.
If all you want to do is athletic activities and be comfortable in the summer, then I'd recommend cacona synthetics -- I've found them to have the best odor/comfort ratio of any of the synthetic fabrics. But when packing for a trip, they come in as runner-up to a good wool shirt.