There are a number of good ones, but first a little lesson on how they work, so you'll understand. Viruses aren't identified by their names, or by what they do (yes, even an app trying to get to a system file may be legitimate), they're identified by patterns. Antivirus companies collect viruses and study them to find a particular string of bytes (maybe a few, maybe a few thousand) that almost uniquely identifies
that particular virus. (Almost, because they all throw false positives once in a while - the code they pick from a virus happens to be exactly like the code of a legitimate app and, until someone notifies them, the AV app calls the legitimate app a virus.)
That said, any company that regularly updates the app's data file is a good one. Which one is best? The one that updated their file with the definition of a virus you're just about to get. And that could be any of the good ones, so here's a list:
Avast Mobile Security & Antivirus
AVG AntiVirus Security
Avira Antivirus Security
Also you should use
Malwarebytes Anti-Malware to catch those apps that are malware, but not viruses. And for ads, popups, etc.,
AdAway v3.2 or
AdWare or
Anti Adware.
I use Avast and Malwarebytes, and have for years, but the others have been around for a long time, I've used them in the past, and they're good. (There's some controversy about whether
Kaspersky Internet Security is safe, so that would be up to you.)