The pair most people skip
Tean Island, split into Koh Tean and Koh Mud Sum, sits just off the southern tip of Koh Samui and is consistently overlooked by guests focused on the islands further north. That's their loss. The reef running between the two islands is some of the healthiest close to Koh Samui, the water is clear, and the whole area has a calm that's hard to find this close to the main island.
Koh Mud Sum in particular has a beach and shallow reef that make for an ideal afternoon stop, easy snorkeling, warm water, and a view back toward Koh Samui that puts the whole island in perspective from the outside.
What to expect?
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Healthy reef close to shore, with good snorkeling that doesn't require a long swim from the boat.
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Calm, clear water sheltered from the main Gulf swell, reliable conditions even when it's choppier further out.
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Very low traffic compared to the northern islands, a quieter and more relaxed stop.
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A short distance from Koh Samui, making it an easy add-on to almost any route without eating the whole day.
On the water
Tean Island is the easiest yes in the route options, close, consistent, and reliably good. It works as a standalone half-day or as the opening or closing leg of a longer day heading north. If the reef is the goal and the day is short, this is where to start.