I can highly recommend Avira.

 

https://www.avira.com/en/index

 


FREE Version is very good
PAID version is very cheap
State of the art detection and protection rating - http://chart.av-comparatives.org/chart1.php
Modest on hardware ressources..... barely noticable even on older systems
Highly configurable..... you can setup in detail how the programm should run and how it treats your security needs. Never again worry about to many alerts or to low security.
Hard to kill. With proper settings the programm withstands most attempts of hostile termination. If you enable password protection, even the settings are protected.
Rock stable: I run the paid version for 5 years... the free versions for over 8 years.... NEVER EVER the programm caused a problem.
Free Version does only display one - Avira - add banner while updating....no other annoying stuff.
Very fast scanner with multicore support.
Developed in Germany..... significantly lower change of NSA backdoor.