As you want to get the details of best course in Degree for IAS study so here is the information of the same for you:

There is no such particular stream which is considered the most appropriate for civil service exam.

Last year results, the maximum number of candidates selected for IAS were holding professional courses

But that doesn't means that you have to pursue a professional course to succeed in civil service exam.

You just need to be focused and have to prepare separately for the civil service exam, no matter which stream you belong to.

As you know that the civil service exam will comprise of three stages:
Civil Service Aptitude Test,
Main Exam and
Interview