There are plenty of alternatives, such as stating Phelps is the most successful olympic athelete of all time or that he has won more olympic gold medals than any other athlete in history.
Winningest may well be a real word (or at least in the US), as in “winning most often” but is a very ugly expression.
There are times when using a single word is the most appropriate way to communicate something, but if the words chosen are distracting they affect the ability of the receiver to concentrate on the message.
Here, I found myself wondering whether or not winningest is a real word, as did the BBC commentators after showing clips of how Phelps’ record-breaking was reported “back home”. As such, it took the focus away from the achievement – and good language should never get in the way of what is being communicated.