A computer without an antivirus is like a Patient with HIV, just like immune system of HIV infected person is getting deteriorated day by day , a computer without Antivirus gets degraded in performance day by day and it not only slows down in processing data but its stored data also gets corrupted or infected and you are no longer able to access your favourite files like music, photos, documents, videos etc.
Although there are so many antivirus softwares available on Play Store or in market but only some companies are trusted when it comes to data security.
Personally I would like you to use Antivirus Software from Kaspersky, McAfee, Avast, Norton, K7 as I have personally checked them.
Although antivirus may make your system a bit slow but in long run it will act as active immune system to any incoming viruses.
You should allow update of antivirus database on every scan as this will help in fighting any virus effectively and efficiently.
I STRONGLY advise you to install a trusted antivirus if you are dealing with offline data transfer from your computer. As offline data is not secure and checked by Internet access servers like Google or Bing etc.
If you are only using your computer for online data processing it has more threat of being attacked by some virus but due to increased security on web Internet access server is itself making you aware about malicious sites and files. So its a bit safer if you are 24x7 connected to Internet.