Science

I think IGNOU is the best one. For one - It was created by the Act of Parliament, 1985. So all its programmes are well recognised by different universities for pursuing higher education and also by industries for jobs.
Getting admission in IGNOU is really easy. They have online admission system. You can simply upload your documents at IGNOU Online Admission and secure admission.
The best part is its study material. They are designed by professionals and Ph.Ds/ experts in fields. So the content is of high quality. Since they are specially made for self-study, they are easy to understand. You receive study material by courier. Infact you receive even exam hall ticket and I card by post and well in time. Or else you can download them from the web site. In short, all processes are simplified. For any other queries, you can seek help from your regional centre.
But IGNOU is for those you are sincere in studies for exams are more difficult than Sikkim Manipal University (SMU) for sure. Also, you need to have submitted your assignments in time. Or else you won't be allowed to appear in that particular subjects exam. In other words, passing SMU exams is a lot easier.
In my opinion, If you want more knowledge and study not just for the sake of having a degree certificate, then IGNOU is your way. But if you want easy way then go for SMU. I am no way devaluing SMU degree, but in my personal opinion, Ignou is better.
Also, you can go to IGNOU regional centre and have a look at study material of your course you intend to study. Or you can download study material from eGyankosh: Home of Ignou free of cost before enrolling and then take a final decision.
All the best!