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.

The nice thing I like about PG's diving is the accessibility of nice easy diving right off the shore. For my first dive in PG, the boat brought me out maybe 100 yards just beyond where a lot of boats are anchored and there were nice beautiful coral heads with lots of fish. After that, I told my divemaster that I'd rather just swim from the shop out to the reef and back so I can look for critters. Around one of the big concrete blocks that act as anchors for the boats, we found 2 harlequin ghost pipefish and 2 baby frogfish.
By far the best diving in PG is on Verde Island. A group of us got the dive shop to rent a larger boat to make the long 45minute choppy trip out to Verde Island a comfortable smooth 30minute ride. Verde Island is a pinnacle that breaks the surface just a little. You can go as deep as you want and constant strong currents make it an awesome drift dive. We started at just about 100ft and slowly made our way up to our safety stop. The largest schools of fish, colorful soft corals and huge barrel sponges make this trip out to Verde Island really worth it.
The other divers I dove with at Verde Island said the conditions were perfect that day. We had 100ft viz and the water was about 78 degrees.
Topside, there is a small town to explore. Massage parlors and bars are available everywhere. The red light district is confined to one alley and easily avoidable if that is not your scene.

The Junk is a very cool boat with a very interesting history including being used in a James Bond film. The cabins are adequate and there are plenty of places on the boat to hang out. The boat is well maintained and didn't have any issues with leaking cabins or broken toilets on my 10 day trip. Food is plentiful and good with a mix of excellent Thai food and some western cuisine. The crew were excellent and a lot of fun, and they brought us to a few beaches to hang out.
One of the coolest things about The Junk is that you get to help raise the sails in the old school way on the last day and go sailing.
Schedules and itineraries vary and they do go to some of the best dive sites in Thailand like Hin Daeng and Hin Muang and Richelieu Rock.

ScubaAddicts has a dive lodge in Umkomaas where you dive Aliwal Shoal. The accommodations are nice and modern. Their boats are well kept and the crew are all very experienced. During the Sardine Run, they head up to Port St. John's and base their operations at Cremone Resort. Food and drink options at Cremone are really good and inexpensive. Maximum 8 divers in the boat with one divemaster and one boat captain. The owners Shona and Craig are awesome.