The healthiest in general would be any that are very close in appearence to wildies (except for colour, although I gues some white rabbits have sight problems). You can't really say mixed breeds or pure breeds are healthier because although a line of pure breeds might have a genetic problem, they could also be incredibly healthy and have no problems to pass on to future generations, same with crossing different breeds, you could be getting rid of health problems that are usually only in one particular line of rabbits or actually adding in problems