Depends if you want above or below the water

Koh Lipe (easier to get there from Langkawi than it is from Thailand - 1.5 hour fast ferry ride from Langkawi although clearing immigration at Koh Lipe can be a CF) has really good beaches. Good for snorkeling and just relax around (not ideal for scuba diving). Most of Thailand's best beaches are now overcrowded save for a few (I've been looking for better beaches in Thailand too)

Pulau Layang-Layang (a pain to get to since it's in the middle of the South China Sea) is amazing for diving and really secluded. Sipadan is world famous for diving but not so much for beach activities.

If you just want a nice secluded beach, Mataking, Lankayan, Pom-Pom islands in Sabah's eastern coast (under advisory from most governments, but kidnappings are rare, more of a precaution issue since these islands have Malaysia naval stations based there) are great. All of these are one island, one resort. The area around the tip of Borneo (Kudat) is excellent (secluded beaches with village life) but typhoons passing north or the southwesterly winds can make the sea rough/turbid.

I've been to Cebu/Bohol and whilst it's not terribly overcrowded, Panglao beach near Bohol Island is not worth the trek (either involving a flight into Cebu, 45-minutes taxi ride from airport to port and 2 hour ferry to Bohol plus another 45 minute ride to Panglao). Flying into Bohol airport (in Panglao Island itself) requires a transfer at MNL which I heard is hell.