This question seems as though it should be easily answered. Unfortunately it's not.
Everywhere I've been has been great though . . .oh how to answer?
I've spent quite a bit of time in Belize and the local diving is OK, sometimes even great. The atoll diving is pretty consistently awesome. Roatan diving is sweet and I've spent a fair amount of time there as well. But for me both of these locales are my "favorites" as much for what the islands offer above the water as below.
Sipidan/Mabul was super and I was fortunate to be one of the last few people able to stay overnight on Sipidan Island itself. This dive trip stole my heart and gave me my first love for muck diving, my first and most incredible hammerhead experience.
The Cook Islands aren't really known for their diving and when I was there the crown of thorns invasion was taking a bitof a toll. But there were white tip sharks on most every dive and we were often close enough to touch. It also delivered my first opportunity to be within the swirl of a huge school of barracuda! Thrilling. And the island is do beautiful, I'd have loved to stay ever so much longer.
Skipping quite a few trips brings me to Raja Ampat. Raja Ampat blew my mind but even so, would I call it my favorite? Maybe if it was easier to get to. I dunno.
Maldives! Oh what can I say? Also not my "favorite" regardless, I was on three different itineraries last January and February and already signed up for a 4th itinerary at the end of this year. So, Maldives might not be my "favorite" but I enjoy it and the diving is a thrill ride!! I can only assume it appeals to the side of me that loves a good adrenaline rush.
Most recently, I ventured farther south in the Caribbean than I'd been before. Grenada Carriacou and Tobago (Speyside ). I couldn't have been more pleasantly surprised at what the locales delivered. My favorites? I dunno, but expect to be back in late fall for at least a partial do over. Woohoo