I would say my personal favorite are the blue acara cichlids, their super reflective light blue coloration just grabs your attention in the tank (especially with dark backgrounds and substrate or planted tanks).
I've liked guppies. I love staring at them for awhile, when I have nothing else to do. I haven't done much in getting a wide variety before in one tank, but these tend to be one of my go to favorites for tanks.
Mine would without a doubt have to be goldfish when it comes to chooses a favorite fish. They are easy to manage, and they look amazing. I guess I am a little biased as I have had more goldfish than any other fish in my time of having fish.
I don't know enough about them to say. Nonetheless, from looking pet store aquariums, I can see they're definitely fascinating. I'd like to get into tropical fish, but the setup to keep them is way beyond my means at the moment.