Earth! All of it. Reportedly, its the world's biggest divesite.
If you want to narrow it down a bit, I've been holding off on the tropical pacific in hopes that my wife reconsider diving...for her, a beach is a beach is a beach.
The Galapagos were amazing...I'd love to go back, but part of the reason why was because it was relatively coldwater. If I'm going to pick just one place, its going to be someplace with cozy-warm water that I can just gently drift around in clear, blue waters, the pure relaxation that comes from easy diving, while being awed by over-the-top geological formations, and flooded by an outrageous biodiversity of critters and reef life.
So where is this place? I can't say for sure. Maybe Palau cuts it, or maybe PNG. I do think that it will be somewhere within that general region, as that's the "epicenter" of biodiversity.
But overall, I find that its not the money that's preventing such a trip - - its the time.
To do a more exotic trip right, you'll want to spend at least two weeks there. Plus you'll probably burn through another 4 days (2 days each way) to go there. As such, you're looking at roughly 3 weeks off from work to do the trip full justice, and that chunk of time is always going to be a factor for working stiffs with jobs, even if someone else pays the $10-20K for the trip. About the only way that it doesn't is if we assume "Lottery" scale cash, for which its $20K for the trip, and then 20 years at $75K/year to replace the job you quit because they wouldn't let you take 3 consecutive weeks off
-hh