Saw bulls in Fiji, now I'm a big-shark-head. Plan to see great whites in SA next summer, so that leaves tigers out of the big three. My total list of want-to-sees runs something like (in no particular order but, universally, spelled poorly):
Threashers
Hammerheads
Tigers
Pygmie sea horse
Any species of shark I haven't seen (horn, wobbeygong, ...)
Humback
blue whale
All-in-all, we've been pretty lucky on sightings without needing to go to anyplace too exotic:
Manta Ray, Big Island night dive, Hawaii
Whale Shark, Holbox Mexico (virtually guaranteed in July and August)
Ornate Ghost Pipefish, Thailand
Manatee, Crystal River Florida, in winter -- freezing cold but frickin' amazing
Seal, do Sea Lions count? If so, La Paz Mexico