There are 2 main beaches that travelers stay at in Ko Samui: Chaweng and Lamai. Some other areas you’ll see are Mae Nam, Bophut and Bang Rak but I did not visit those areas. If you’re arriving by ferry, you’ll probably arrive in Nathon but you don’t want to stay there unless you have to catch an early ferry out. There is little for tourists to do there. From Nathon pier a songthaew to Chaweng or Lamai should costs about 100 Baht.
Chaweng is most developed beach on the island and has the nicest beach (in my opinion). Chaweng beach is a pretty long stretch of beach. The busiest part of the beach is near Ark Bar. The areas north and south of here are less crowded, with the south part a little more quiet. I walked up and down most of the beach and the resort with the best set up and most customers was Ark Bar.