I have 3 recommendations for you:
1) Sipadan Island off the East coast of Borneo
2) Sipadan Island off the East coast of Borneo
and last but not least
3) Sipadan Island off the East coast of Borneo
If you go to Sipadan Island you will see so many turtles in the first 2-3 days that you will become bored with them. Sometimes it seems like they are so numerous they get in your way. They are so accustomed to divers (no touching or riding allowed) that they are absotulely fearless and on occasion I have had to yield to an oncoming turtle because he (or she) figured they had the right of way. I have seen turtles on Guam, Palau,Yap, Hawaii, Christmas Island, Okinawa and Sangalaki but no where have I seen them like on Sipadan. We were even lucky enough to to have the voyeuristic opportunity to observe 2 turtles in "the act".
Here is a picture I took on Sipadan of a turtle balancing on a small coral head while taking a "siesta".
And the good thing about Sipadan is after you get tired of the turtles there is a multitude of other things to see also.
Rickg