AV-Test Android Test September 2015

There's not a lot of difference between the vendors, except for features, so I personally would not pay for any Android antivirus apps. A very basic app can provide decent protection against unwanted apps. I have recently installed Bitdefender Free on my smartphone, it scans when I download apps from Play Store, all without any bloatware or nag screens, as I have previously experienced in other free apps.

Again, antivirus for Android is optional.
 
@Huracan Just as an AV on a Mac might be optional an AV on Android is as well since Google implemented permissions for example
when you first install a certain app :) On OSX it will ask you if you really want to open a downloaded file/software