a few years ago i had to pare down my wardrobe for extended travel. sadly (at the time), my startup tees were the first to go. i replaced them with patagonia's silkweight tees (2/3 merino, 1/3 synthetic).
as a lazy hacker i won't go back to cotton. i can comfortably wear a 2/3 merino + 1/3 synthetic base layer in virtually any climate. unlike cotton or pure merino, my pits don't remain wet after sweating. and unlike 100% synthetic tees, i don't smell like a locker room by lunchtime. (lazy bonus: this means fewer washes in cooler weather.)
there are a couple of downsides though:
- they're more expensive (~$40 on sale)
- they're delicate (delicate wash, and tumble dry with no heat or lie flat to dry is recommended)
there may be other companies with similar-quality products, but i'm comfortable recommending patagonia based on my experience. with proper care their stuff should last a long time, but they have a great warranty just in case.
[edit: formatting]