What is a good CGPA in Anna University?

There is nothing called a good CGPA. CHPA depends on what you are willing to do afterwards. 

For a job: As far as placements are concerned, anything above 7 or 7.5 will fetch you a decent job. 8+ will get you even better jobs in flagship companies.

For higher studies: If you want to pursue higher studies, much more is considered than just CGPA. Some of these factors are 10 and 12 marks, GPA and work experience.

So an 8+ GPA will get you both a good job and colleges. But it’s not the sole factor. It all comes down to how much efforts you are willing to put.