Nadal's story would be most interesting... the weird dynamic of his camp, all the injuries and big gaps in his career. Not to mention the dominating influence of Toni - it'd have to potential to read like the story of someone who escaped from a cult. But Nadal probably wouldn't be able to produce a good autobiography about it, even with help. And it'd probably never happen anyway.
In reality I think Murray's would probably be the most interesting. He produces the most boring demeanour imaginable in some interviews, but away from that he is much more interesting personality. He also comes across well in writing, with the ability to be both funny and deal with more serious issues.
I think Djokovic and especially Federer, unless they have any big secrets they could reveal, would be a little more boring. Federer's professional and personal life have seemed like pretty smooth sailing, and I think that even beneath his PR image he is just boring and sensible. Plus, I think Murray's struggles to win a slam and eventually Wimbledon would make for a better story than Federer and Djokovic's more uber-successful careers. "Oh yeah, this was the year I won my fourth Wimbledon and made YE #1 again... it was pretty tough."