For me it's as follows:

1. Radix - just for sheer boldness, calmness when handled, voracious feeding...oh and I think mine are beautiful.
2. Infernalis - a species I keep coming back to again and again. The most enthusiastic (put politely) feeders I've ever had.
3. Concinnus -I love their red heads. That is all. Simples.

The others that I have kept all had their querks etc and have been fun to own but the three above just do it for me...

I may well add atratus to this list after Doncaster if John's experiences are anything to go by.