The funny thing about the best place to go diving is that once you go there, you find out that there's someplace better!
If you are looking for pretty diving (soft corals, fish, good vis) then Fiji is a wonderful place. There are several live aboards and land based options. I can vouch for Naia, it's a great boat and you go places that you just can't reach from land based operations. I've also been to Cousteu's Resort. The property is pretty, the house reef is so so, but if you are up for longer boat rides, the diving improves. Beware of the mozzies here though.
If you are thinking of cashing in air miles and heading to Australia, the GBR is a great place. You really should do a liveaboard here in order to get away from the day divers out of Cairns. Mike Ball and the Nimrod are good choices. Good diving, fun critters and if you go soon, Minke whales are migrating!
If you are up for an adventure, try Exmouth and Ningaloo Reef. You'll be diving in the Indian Ocean about 500 miles north of Perth in Western Australia. It's a great place to see lots of hard corals, schools of fish, Mantas and Whalesharks.
If you and your family are experienced divers, then there are some really great places in Indonesia (Lembah Strait, Irian Jaya, Bali, Komodo, Banda Sea) as well as Thailand (Phi Phi, Andaman Sea).
You can combine diving with nature walks by visiting the Galapagos Islands. Diving is challenging, lots of currents. But... it is amazing.
If you want to stay closer to home, try Grand Cayman or Cayman Brac. Good walls, nice Caribbean diving.
Sigh... so many oceans.... so little time.
-Rob