MSE paired with Malwarebytes is effective and stays out of the way, and what I use
All of the others listed so far are pretty good as well, but tend to either be larger programs, or run annoyingly in your face, or always try and get you to pay for the full version.
MSE and MBAM play nice together, take a minimum of system resources, stay out of the way until needed, and they each fill in the gaps that the other is missing (MSE is weak on anti-malware, and MBAM is antimalware that only scans for a few viruses).