what is a good outdoor shirt?

Should it be made of 100% synthetic material or 50% synthetic?

What are your thoughts about this issue?

for Florida heat and humidity when I'm in the Sun all day... I use 100% nylon (preferably DuPont supplex finish) in a long sleeved vented cape back "flats fishing"
style...typically "salmon" color to reflect as much IR as possible... with loops to tie up the sleeves if desired...for less prolonged exposure a fully synthetic "wicking" polypropylene ... generic UnderArmor

I get great results with Nike fit and Nike sphere tops. They wick away moisture very well even when I have a backpack on.

No affiliation except a happy customer.

Layers.

I tend to wear a Mountain Hardwear T-shirt (mostly poly something) over or under a Patagonia 3 long sleeved shirt made of Capilene. I think that Capilene is the bomb, I have worn the same shirt for 5 days and 79 miles with no complaints from fellow travelers - no worse than them and downright petunia smelling compared to a few. Really though, Patagonia wears well and is comfortable and really doesn't make me smell after days on end. When it gets wet you stay reasonably warm until it dries.
Now is the time to search out Patagonia discounts on this stuff (and other base layers and winterish clothing) - Mountain Gear in Spokane mountaingear.com has a sale going on.

No affiliation etc, just a happy customer.

Outdoor shirt is fairly vague. So I will post a couple. I have a big fan of under armour loose fit short sleeve. My preference it that they seem to fit my body type well which make me more comfortable wearing it when i sweat and it clings. YMMV.

My absolute favorite outdoor hiking or adventuring shirt is a decade old Columbia LS button up shirt. Very similar to Ex Officio or current Columbia shirts. Very light, the sleeves provide mosquito and sun protection but roll up. With a pack on my back I can vent by opening buttons.

A shirt that I like to run with in high temps for long distance is Rail Riders Eco-Mesh Adventure Shirt. Man that thing looks good, breaths great, sun protection. When it gets soaked it dries fast but being super thin it clings skin tight.

Long sleeve, two pockets w/ button flaps (so goodies will stay in them), either all synthetic or blend (varies with the brand). Team up with a good wicking T-shirt (capilene is great for that purpose).

That pretty well describes a basic, versatile shirt. Layer up for cold, blustery and stormy conditions; roll up the sleeves for warm days. In really hot weather, you might go with an all cotton, loose fitting shirt that will provide sun protection.