How is NIT Arunachal Pradesh? Should I choose it over a local but good private college - Heritage Institute of Technology, Kolkata?

If you have a decent rank, you should prefer another college than NIT Arunachal Pradesh. At NIT AP, you will be studying with 6–7 lacs students as well with Home state and reservation quota students which can feel partial at times. 

Highlights of NIT Arunachal Pradesh:

• Average curriculum: The faculty member at NIT Arunachal Pradesh lacks knowledge and qualifications. The course curriculum is average. There is also a lot of bias.
• Best Department: Computer Science and Engineering, Electronics and Communication Engineering
• Worse Department: Electrical Engineering in NIT Arunachal, no quality
• Infrastructure: The infrastructure of NIT Arunachal Pradesh is not so good.
• Job Security: The placement cell of NIT Arunachal Pradesh is the only good thing about this college. If you have opted for on-campus placement, the probability that you will get a job offer is close to 100%.

The placement record of NIT AP is exceptionally good and you can rely on their placements. So if job security is something you aim for, go for NIT AP.