Bamboo and micromodal make good underwear. I haven’t tried them in the summer yet so I don’t know how they hold up in heat and humidity.

Linen can be good for underwear. It’s very breathable, but sometimes it takes a few washes before it feels soft. Also, it doesn’t have any natural elasticity, so it needs an elastic waistband to stay up. Sometimes it has a drawstring, but I prefer the waistband personally. It’s also loose and flowy, so if you wear tight clothes and don’t want it bunching underneath, linen isn’t a good choice for underwear. If you wear loose, flowy clothes anyway, linen is a great choice.

If you’re willing to put out the expense, silk undies sound kinda awesome. Never tried that myself though so I’m not sure how breathable it is.

Personally, I love linen in general for hot weather. You can wear long sleeves to keep the sun off, and its so airy that you don’t even feel hot under your clothes. Especially if it’s a light color. It also dries quickly, so it’s not like cotton where once you sweat you’ll be wet all day. Spend thirty minutes in the shade on a breezy day, and your linen clothes will already be dry