I don't know I love all my fish equally I can't really have a favorite because they are so cool. Silver dollars, funky catfish, pleco, gouramis, mollies everyone has their points it wouldn't be fare just to pick one.
I pick betta's because I've only kept bettas and guppies out of that list, and I'm not having such a grand time with the guppies while the bettas were my babies.... besides, bettas are mini meat eaters, unlike those guppies and their infernal flakes and occasional shrimp pieces...