In agreement with the above posts, Holden is the biggest 'phony' of them all. He is super-critical of the faults of others, but is completely blind to his own short-comings. He continually labels the people he encounters with derrogatory nick-names or slurs, but fails to see how his own narrow-mindedness makes him the biggest 'phony.' Holden sees his insight as being original or perceptive, but it really just comes across as sour-grapes negativity.
Don't forget what we know about Stradlater comes from the very skewed perceptions of Holden as the first person narrator. Yes Holden seems sincere about Pheobe and Allie, but he is phony in the sense that he criticizes other's short comings, but he has excuses for his.
In The Catcher in the Rye, it would be easy to say that Holden is the phoniest, since--as narrator--he has the chance to be the most hypocritical. But, I like that way Holden rebels against society and calls people out: he sees through the deep-seated moral problems inherent in the modern world. And, Holden sincerely wants to protect his sister and the memory of his brother, so I'll pin the blame elsewhere.
Most of all, Holden denounces materialists and people who "play act" in order to get approval from others. He has a litany of phones: glad-handing adults (the headmaster), over-sexed teens (Stradlater), pretentious snobs (Luce), the celebrity-obsessed (girls in Lavender room), and materialistic artists (D.B.).
The biggest phony, for me, is Stradlater. He seems like the epitome of a hypocrite, a big-man-on-campus who refuses to do any work. Instead, he subcontracts it to Holden. And after Holden writes him a touching essay about his dead brother's baseball glove, Stradlater gets sore and beats up Holden. Sure, Holden provokes him, but it doesn't take much. Not only this, but Stradlater moves in on the only girl Holden has feeling for, Jane. He seems out to score with her simply out of spite.
I think that this may be cheating, but I would vote for Holden as the most phony character in the book. I say that it is cheating because he's the one we know best and I think that the more you know someone the more phony they are likely to seem.
I say he's phony because, for example, he is constantly saying how phony other people are. But he himself does not really act a whole lot different. He says other people are phony, but he still tries to be friends with them. He hates Stradlater but will do his paper for him. He thinks the women from Seattle are pathetic but he wants to dance with them. He says the Lunts are phony but he'll go and watch them.
So he's criticizing society all over the place, but he does not withdraw from it. In fact, he seems to be trying to be involved even as he is saying how phony it is.
On that subject, why was he the manager for the fencing team? He hates jocks but he's the manager for a team? Sounds phony.