There is only one person on this list who has done something no sportsperson has ever done before. Twice. In the same Year. He won the Tour de Romandie, Paris Niece, Dauphine (never done before). Then the Tour and and Olympic gold (also never done before). Wiggins. More medals than any other British Olympian ever.
All the others have done astoundingly well. The other outstanding achiever on this list is David Weir. Come on. 800, 1500, 5000 and Marathon. What amazing versatility and dominance. Far more challenging than Heptathlon. Sarah Storey was amazing, but I think David Weir has a lot more close competition, to be honest, as his sport has more participants and is a very mature sport. And he gets overlooked. A lot.
If it has to be a female athlete, then it has to be Sarah Storey.
IMHO