My favourite is Chela.
But you're not asking that. Well, Gaudio is the only one out of them that has won a Grand Slam. But I don't think he's the best in the group. If we're talking about at their peak, I would say it's a tie between Nalbandian and Coria. Canas, when he is in form, is also extremely dangerous.
If you're asking who is the best right now, I would say Nalbandian.
Basically, in my opinion, all of them are excellent players, some are just more accomplished than others.