Lightweight merino or merino/poly blend irrespective of weather for me- when it's seriously hot and dry, threadbare cotton is my go to material.
I haven't had much luck with 100% nylon or poly garments, especially in direct sun and heat, Capilene 2 being the exception- that material is a wonder fabric thanks to the weave Patagonia uses. I soak out my back wearing merino, even in cool conditions, but I don't mind it one bit. A damp merino shirt feels significantly more comfortable on my skin than a saturated synthetic number.
The texture, weight, and knit pattern is equally as important as the material, having owned a few merino hoodies I have learned that not all hoodies are equal despite equivalent fabric weights and material. My old BPL Beartooth Hoody was fine into the 80's, the material was thin and handled the broadest range of temperatures of any garment I can think of.
Consider a thin merino/poly blend for hotter days, and when its really hot and dry (90's-100's) don't rule out a simple cotton shirt.