Sorry, I did not see your message about only voting for one until after I voted... (opening messages should be at the top of the poll!)
My favorite types of fish are wrasses, gobies and butterfly fish.
If I had to only pick one fish, it would be the Dragon Wrasse - probably the most interesting and interactive fish I have ever had (and that's saying something since I've had fish/aquariums for almost 30 years...
Although, after studying Animal behavior in college, I have to say a close second would be the barracuda that House of Fins had in their marine pond. Crazy smart fish and super interesting. I plan to build a marine pond at some point just for a barracuda...