Mango Bay is on the north side of Koh Tao. Among the places I've dived on around Koh Tao so far, Mango Bay is far and away my favorite. The landscape of the sea bed offers a seemingly infinite variety of large coral boulders to swim around, over and between. Passing each one reveals a new array of fish and corals.
Mango Bay is such a wide-ranging site that it would be easy to get lost. This is probably a good site to practice your navigation skills - see how close you can get to your boat after a long look around.
The fish to be found includes shimmering gold damselfish, angels and, if you settle onto the sandy bottom at just the right spot, a wrasse will come and clean your ears for you.