Ibex Woolies Pro Tech Crew and matching Woolies Pro Tech Bottoms ($115) are the best lightweight merino base layer I've tested. They've been the top pick in our merino wool clothing guide for years, and I struggle to imagine what would unseat them.
Woolies are Nuyarn, which is 85 percent merino wool and 15 percent nylon. The merino wool is wrapped around a nylon core, which increases the warmth while being lighter (5 ounces for the men's large) and more durable. Woolies are 125-gsm wool (that's grams per square meter, a common measurement of fabric thread density, and therefore relative warmth). The cut here is tight, which is part of what I like about the Woolies—it's easy to get your next layers over them, and they don't bunch up. There are generous thumb holes on the sleeves which make it easy to pull on additional layers without the sleeve riding up. The flatlock stitching makes it so I don't notice the seams (which are mostly off the shoulder anyway, to prevent any rubbing should you be wearing a pack).
I've been wearing Woolies as my main base layer in winter for over three years now. This is also my top pick for all things technical—backpacking, bike packing, climbing, skiing, etc. It's what I bring hiking and backpacking to elevation in the summer months as well. Paired with a merino grid fleece and lightweight puffer jacket, this is even enough for shoulder-season trips in the fall. I prefer the crew T-shirt since I usually have other things over this anyway, but there is a quarter zip version of the shirt if you prefer to have more ventilation options. If these have a con, it's that they frequently sell out in popular size and color combos. (They also restock frequently, so keep checking if the one you want is not currently available.)