There are definately waaaaaaaay to many to choose just one...but I could narrow it down to a few.
Clown Trigger, Queen Angel, Volitan Lions, Dragon Eels and Dragon Wrasses.
Well by sales volume and popularity i'd say three fish would have to be the most popularily kept. the yellow tang, percual clownfish and blue hippo tang. it seems most every tank i see has at least one of these fish!
Since i haven't keep that many different fish, i don't have many different ones to pick from.
so far, clowns and yellow tangs. though later in life when if i have others i may have a new favorite. the clowns require little manitance and are hardy (which is important to me as a college kid)
My favorate fish is the blue hippo tang. I love the color on this fish. It gives out great beauty of your tank. The sailfin tang gives out a design of your tank that is my second favorate.
I would have to say anything not readily found in other peoples tanks. I know, I know, I have a coral beauty, but it is going back to the LFS soon. I love my Marine Betta even though he hides alot, and the blackcap I just got is way cool.