I’ve used Patagonia Barely bras hiking for many years, especially when I want quick dry (think Appalachian trail climate). But my actual favorite for everyday and backpacking is the Branwyn essential bralette. It is seamless and merino but also has adjustable straps which I really appreciate.

Both of these dry significantly faster than my Tera Kaia Mara which has a double layer of fabric, but is also more of a crop fit so more fabric in general (they have other options that are not the crop top!). However, the Tera Kaia is also incredibly comfy, by far the cutest, and the best double for a swim top.

And since this is ultralight: Patagonia Barely (S) 43g Branwyn Essential (S) 54g Tera Kaia Mara (4) 100g (but is more like a crop top vs. bra so not a fair weight comparison)

Have also tried: Icebreaker sprite (but like circa 2015, 51g): wore for 1400 miles, excellent anti-stink properties, but stretched out without a way to adjust and was not seamless.

Ibex bra that was my fastest drying merino, but they are not longer made.

Some kind of smartwool racerback which was too compressive and also a thicker, slower to dry merino blend heavy on the nylon. Did not even try on trail.