The fastest way to eat badly in Koh Samui is to only eat where the tourists eat. The fastest way to eat brilliantly is to ask someone who lives here. Hello.
The island runs the full range, from plastic-stool street food that turns out to be the best meal of your trip, to candlelit dinners with a view that costs accordingly. Both have their place. What you want is to know which is which, and which spots are coasting on the view alone.
Below is the honest version: where to find proper Thai food, the restaurants where the view actually earns its price, and the local places that never make the tourist lists. Eat like you live here, even if it's just for a week.