well, lets see here. this is going to be kinda hard to narrow down do to me liking all species. but i'll try to give it a go at.
Catfish: Stripped Raphael
Pleco: Royal, Clown, and Zebra
South American Cichlid: Discus and Angels
Central American Cichlid: Firemouth
Rift Lake Cichlid (including Victoria): ??
Tetra: Cardinal and Neons
Barb: all Tigers
Gourami: Powder Blue
Oddball: Silver Arrowana
and the "ALL TIME FAVORITE" i would have to say the Angelfish, i have kept them the whole time i've been into housing fish.