The Perhentians are the northern-most of the island groups off the east coast of Malaysia. There are nine islands in all in the group, but only the two larger islands are occupied. Both Pulau Pernetian Besar ("Big Stopover Island") and Pulau Perhentian Kecil ("Small Stopover Island") have small towns with many accommodations to choose from. The islands have become quite popular with tourists, including many non-divers. Indeed, by some accounts they've become a bit too popular, with reports of degraded conditions at some sites. Still, recent divers report some great dives, so conditions aren't that bad.
I've highlighted just a few of the dive sites around the islands on the map below. These seem to be considered the best, although a lot will depend on conditions when you visit. Note that some resorts close in the low season, between November and February or so.