Which is the best job in computer science among web development, software engineering mobile and android development?

All of them are great options. You should select in which you are good at. I believe Web Development and Software Engineering will be the best to select.

Among these three, web development is the best job for CS graduates. Moreover, all these are best in today's world but after few years Artificial Intelligence, Machine Learning, Data Science will overcome these jobs.

All three are the best in today's world. Software Engineering and Android development are really good field to pursue though. Though it's completely your opinion to what interests you.