The marine life here is incredible. No wonder this is a top rated dive destination!
My favorite dive site was Barracuda Point. There were constantly large schools of jacks and schools of barracuda every time we went back...and we kept asking to go back.
Turtle Cave was a cool dive site. It's where turtles go to retire and there are turtle bones and shells even at the entrance of the cave. Eerie but different.
Hanging Gardens is not a "famous site" but it was really beautiful with all the colorful soft corals.
There were lots of black tip, white tip and grey reef sharks and lots of turtles on every single dive. I even spotted a hammerhead on my first dive.
Most dives start deep > 100ft so get yourself nitrox certified and dive with it along with a computer to extend your bottom time. Even in the winter it was warm in the mid 80s and I think the worst viz I got on my trip was 60ft. Most dives was over 100ft viz.
Book early and let the dive operator know that you want to dive Sipadan coz a limited number of divers are allowed on the reef per day.