Choosing where to stay in Koh Samui is the one decision that decides what kind of holiday you get, and most people make it blind off a map.
The island isn't one place. It's a string of very different beaches, and booking the wrong one is how you end up with a nightclub under your balcony when you wanted a hammock, or the other way round. Chaweng is the big night out. Lamai is its cheaper cousin. Bophut is the grown-up dinner. Chaweng Noi is the quiet one (that's us).
We live here, so below is the area-by-area version with no resort trying to sell you anything: who each neighbourhood is actually for, and which one matches the holiday you're picturing. Start with whether you want the noise or the quiet, and work from there.