I use the Outdoor Research Sequence Duo T in cool weather, and have never had an issue with itching due to side seams - but I don’t know if that’s due to construction of the garment, or just my own personal sensitivity level to such things. The bad news? They cost $50.

In the 90-degree, 90% humidity of an Ohio Valley summer, a merino-blend anything is just too hot, so I use the OR Echo series T-shirts. A bit of a used-Kleenex feel, but they do a good job of moving moisture away from me and letting it evaporate (or what passes for “evaporate” in such conditions.)