Merino wool shirts

Yes, $90 for a T-shirt seems insane at first, but hear me out: Merino wool is amazing stuff. It keeps you cool when it's warm and warm when it's cool. It hardly absorbs odors at all, so you can (and should) wear it a dozen times or more before washing it. That means that you really only need one or two (and if you compare, say, $180 for two great merino T-shirts versus $210 for a week's worth of mediocre $30 cotton T-shirts that require doing a lot more laundry to boot, the price doesn't seem that outlandish anymore). It also means that it's perfect for traveling light (I was in Italy for 10 days recently and I literally had no shirts in my bag, because I only took the merino T and merino long-sleeve I had on and just wore those the entire time). And it's great for workout wear, because you can sweat into them as much as you want and they'll never get that long-term mustiness that cotton gets after a while even when you wash it.

Wool & Prince's 100% merino wool T-shirts are my favorite for daily wear. Icebreaker is more athletically oriented and I like both their T-shirts and their base layers.