I have been to Langkawi, but not to Phuket and Koh Samui. I went to Langkawi at a time when I had had no experience travelling in SEA. It was my first trip to that part of the world. At that time also Phuket was better known than Langkawi. Even people who had never been out of the U.S. asked me if I went to Phuket, after I came back from my trip to Singapore, Malaysia and Thailand. Since then, I have thought of going to Phuket or Koh Samui. I found Koh Samui to be overpriced and Phuket overrun by western tourists. My suggestion would be Langkawi.
Since then I have looked into going to Phuket or Koh Samui. I think, both, Phuket and Koh Samui attract more tourists than Langkawi.
I rented a small motorbike and road around the island (I was backpacking). You will have to get away from the area with concentration of hotels in order to enjoy the island.