There’s nothing worse than getting rubbed raw in your nether regions, which can happen (especially on hot days) on long hikes, especially if you’re wearing the same cotton briefs you wear in your non-hiking life.
Synthetic, boxer-style briefs fit snug against the bod, help wick moisture away from the skin and prevent your inner thighs from rubbing against each other. They work well for me. Others skip undies altogether and go with Spandex tights.
Whatever you go with, mind the chafe, or it’ll bite you. In a place you don’t want to be bitten.