I had always assumed it would be Elizabeth of York but if her brothers had been declared illegitimate that meant she was as well. If not her, who?
If Richard III's supporters had won at Bosworth, but he had still died, then the most likely claimant would have been John de la Pole, earl of Lincoln, as he was Richard's nephew by his sister Elizabeth, and was his probable chosen heir after the death of his only legitimate son in 1484.
If Henry VII's supporters had won, then they would have probably rejected de la Pole as being too closely linked to Richard III. Instead, they would probably have favoured Edward, earl of Warwick. He was Richard III's and Edward IV's nephew by their brother George, duke of Clarence, but had been largely rejected by Richard, kept under house arrest, and prevented from assuming his titles and lands. Edward IV's two sons, the Princes in the Tower, were presumably dead by the time of the Battle of Bosworth in 1485 and Richard III's only legitimate son had died the year before, leaving Warwick as the sole remaining male heir of Edward IV and his brothers.
The legality of Warwick and de la Pole's claims are uncertain. Warwick's father, the duke of Clarence, had been attainted and executed as a traitor, something that normally barred the traitors' heirs from inheriting their parent's titles and claims. However, considering the political confusion that would have arisen if both Henry VII and Richard III had died at Bosworth, this uncertainty would probably have been overlooked in the panicked search for a new monarch. Warwick's claims were clearly accepted by some people, as it was his identity that Lambert Simnel adopted when he tried to raise a rebellion to overthrow Henry VII.
De la Pole's claim was slightly stronger than Warwick's as his parents had not been attainted. The de la Poles repeatedly posed a threat to the Tudor dynasty. John de la Pole participated in Simnel's rebellion against Henry VII, his brothers Edmund and Richard de la Pole conspired against Henry VII and Henry VIII.
Sources:
Michael Hicks, The Wars of the Roses (London, 2010).
Rosemary Horrox, Richard III: a Study of Service (Cambridge, 1989).
Charles Ross, Edward IV (London, 1974).