Philosophy?
Psychology?
A "hard" science?
Mathematics?
A general liberal arts degree?
I think it depends on the type of critical thinking to which you are referring. If you want to talk about reading a paper, essay or listening to a speech, then I'd say Philosophy, since it teaches you to look at ideas and presentations logically in an essay format.
If you want to talk about analyzing data, impersonal facts, theoretical models, graphs, and the like, then I'd say Mathematics since it teaches you to look at these things at their most fundamental level, and it also makes you focus on the process of reaching a conclusion.
If you want to talk about bridging the relationship between an abstract theoretical view and the real world application, then again I'd say Mathematics (assuming a person also studies how to do proofs), since Mathematics starts at the most basic theoretical foundation and takes you all the way to how these ideas can be applied in a more concrete way.
If you want to talk about critical thinking about events in the world around you, then I'd say forget school and try starting your own business. You'll learn the hard way what works and what doesn't, and you have to make these decions in a wide variety of ways: financing, customer relations, time management, etc....
So overall I'd say in a essay context Philosophy prepares you the most, in an analytical or problem solving context Mathematics is king, and in a purely real world context running a business prepares you the most. Other than that I think you can learn a lot from the school of hard knocks.
