I don't know about hardiest, but in my starter discus - 3 red turqs, 3 RSG, 3 blue diamonds and a Marlboro Red, I still have all 3 red turqs and they are big and healthy...
I also have one blue diamond. I ended up culling the RSG (except one who died), the marlboro and the other 2 BD. Some didn't grow well (I admit to some overcrowding) and some didn't end up meeting my standards.
The red turqs definitely seem tough and forgiving.
John