I use Avast, it's very good. The downside is that on Windows 10 the NG is not ready. Avira is also good but it lacks 0-day protection, that is a big downside to me.
For paid security Kaspersky, ESET and Avast are in my top 3
When it comes to Memory Usage / CPU depends on what PC do you have. On my PC (Windows 10, 64x) Avast uses between 25-30 Mb.