If I had to choose only one device from mobile phone, computer or fridge for using I would choose a computer. I can explain why.

Firstly, I can use it for calling people through the Internet as well as mobile phone. Secondly, I can find any information I need and use computer for spending leisure time. For example, I can read a book, watch a film, play games or listen to music. Thirdly, I can buy all necessary things in online shops. It is really comfortable, because there is a huge assortment of things and it is easy to find what you need in these shops. Moreover, I can order different fresh food which will be delivered to my house for a short time.

To sum up, I would choose a computer as the most necessary device. Of course, it has one meaningful disadvantage. I will not be able to take computer everywhere because of its size. However, I think it is not so important, because only computer let me do much work using different programs. Mobile phone and fridge don't have such possibilities.