I will personally recommend Bitdefender Mobile Security as it was the only mobile security I kept until the very end of the trial without uninstalling. It has a very non-intrusive browser protection, anti-theft, and malware protection. Avast is very good as well, but I did find it to be a bit heavier on the device, and a bit more intrusive, thus having more battery drain.
I have tried CM Security and 360 Security as well. CM Security is bundled with the Clean Master suite, which is an automatic no-no for me. 360 Security, although the #1 AV on the Play Store, has been known to have a somewhat shady reputation, whitelisting malware for companies that pay them off.
Bottom Line: Bitdefender Mobile Security
Edit: However, since the trial ran out, I have been using Bitdefender Antivirus Free as mentioned in the post below by
@Huracan. It is more than enough. Even if the cost for the paid version is low, the device bulk and battery drain will increase for features I do not necessarily need.