I want to know the best university for a PG diploma in computer application.?

Since you have not mentioned the country where you are looking for universities, here is a list of top universities that provide PG Diploma in computer applications across the globe:

University of Oviedo
Universidad de La Laguna
Monash University 
University of Leon
ISCTE- University Institute of Lisbon 
Vrije Universiteit Amsterdam
Aalto University
Malardalen University
PGDCA is a prominent programme in the field of computer applications. However, the field also provides other courses under various titles, such as:

Masters of Information Technology 
Master of Smart Systems Engineering 
Master in Applied Linguistics
Masters in Computer Science: Parallel Computing Systems 
If you have any further queries just ask me.