Definately Hin Daeng and Muang, you don't need Mantas to have a great time here. I think the Similans only just become accesible in November, If I remember right, they are unreachable from May to Nov but if you went a little later, Similans and Richeleau Rock are well worth a visit.
I'll be down in the southern andaman myself late December and will be checking out Ko Ha, I've heard good things about it but never dived it myself.
Wherever you go, don't go to the Gulf of Thailand at this time of year, 5m swells and vis of 2m is not uncommon. Stick to the glorious in season Andaman side.
Personally I enjoy staying on Phi Phi, Hin Daeng/Muang are only a short speedboat ride from here.