Which degree is suitable for me to be an IPS?

To apply for IPS, you only need a UG degree. It can be of any field, Arts, Science or Commerce. I think you should go for BA as you will get lots of time to study and prepare for the IPS exam.

Any graduate from a recognised university is eligible for IPS, they do not ask for any specific degree, but the eligibility for IPS is a graduation degree and it gives 6 attempts to the general category students till 32 years and to SC/ST category till 34 years. Good luck.