For the most part, as long as you use app stores from either Google or Amazon, you should be fine. However, if you want a little extra in case of the remote possibility, I like AVG.
I've used Avast! Mobile security on my Nexus 5 for over a year and now on my Tab S. It's totally free, but they have pay options for other features. The web security runs in the background, scans any downloads and apps installed. Doesn't seem to tax the battery or cause any problems from what I can tell.