Been to Fiji and the Philippines, snorkeled in Tahiti and Moorea (too young to dive at this time). Let me put it in pros and cons.
Pros :
Fiji : unbelievable colors underwater, soft and hard corals to die for, I don't remember a dive without a shark, friendliest people in the world -honest you'll shed some tears when they're going to sing "Isa Lei" on your departure-, global cultural experience, kava drinking.
Phils : most incredible UW variety of the lot, can be dirt cheap for diving, inexpensive lodging and transports, San Miguel beer is less expensive than drinking water, that asian feeling, 2nd friendliest people in the world : after two days, you're part of your dive guide family.
French Polynesia : lots of pelagics, sharks and rays, blue water viz, french cuisine, probably the best beaches of the lot (although Fiji Yasawas are a serious contender but with tame diving).
cons :
Fiji : hmmh... a little costly maybe?
Phils : Let me think... Ha, I don't like their food, except grilled fish, not spicy enough.
French Polynesia : Horribly expensive, that french divers arrogance sometimes (disclaimer : I AM french...
), dead coral bottom, less fish variety than other places.