Let's separate reef fishes from pelagics.
On the reef :
Merlet's scorpionfish, looking at the glossies, the most beautiful reef fish IMHO.
Mimic Octopus.
Looking out to the blue side :
Whale shark, I'm looking for a place you can spot them just snorkeling to bring my daughter with me.
Tiger shark for the "pinch" (preferably on a night dive. Hmmm, no finally not, let's stick to day dive...)
Mola Mola, so...different...I suppose you cannot mismatch it with a trumpet fish or a barracuda, even schooling...
Marlin : I dived twice on the same spot in Makassar where the DM spotted two days before a big marlin. Though he had been diving for 10 years in Indonesia he told me he hadn't seen anything as impressive underwater. We stood there waiting for the show but unfortunately, the marlin did not show up.
Note I would not mind spotting again threshers (half way between white shark -the snout- and panda bear -the eyes-, ok may be not the panda bear but still very particular eyes...), manta rays, eagle rays (most graceful thing I've ever seen), pygmy seahorses (remember the old theory that in every atom there's another galaxy, made of atoms in which, etc. same feeling when you spot a pygmy seahorse, when your eyes get accustomed to the size, you then usually see 2-3-4 and more on the same gorgonian fan) , ghost pipefishes (THAT'S weird, man!) or even squids (closest thing to "encounter of the 3rd kind" when they start changing colors).