Even if Pizzi was to agree to the deal, San Lorenzo would also have a word in it and Valencia CF would need to pay them for Pizzi's contract cancellation. The amount isn't too big, but it can be a legal problem to deal with.
Close second if the deal with Juan Antonio Pizzi fails is Marcelo Bielsa, but on the short list there are other coaches like Pepe Mel, Villas-Boas, Miguel Ángel Lotina and others.
Personally I'm not too fond of anyone if particular, if anything Pepe Mel could surprise and do really good with Valencia CF, Bielsa would be the default choice as a really experienced manager, but he is still a risk with his style he brings.
Guus Hiddink is also a free coach, though he hasn't had much success recently, so a distant prospect if other options fail.