Which exam is best for MBA?

CAT is considered the best entrance exam for MBA in India. CAT is accepted by 20 IIMs, and also many eminent Non-IIM b-schools (such as FMS Delhi, SPJIMR Mumbai, MDI, etc) across the country.

If you want to pursue an MBA from ISB, or other b-schools abroad, the GMAT is considered the best exam.

Apart from CAT and GMAT, other popular MBA exams (for MBA in India) include:
XAT, IIFT, TISSNET, SNAP, MAH-CET, CMAT, etc.