It would be hard to just have one module/component installed, what is it ?
I would say that the Web Shield is perhaps the most important shield, given that the internet is the most likely attack vector so needs protected and the File System Shield a close second. The Behaviour Shield provides another level of protection as it is monitoring behaviour, something may not necessarily be detected by other shields but exhibits suspicious behaviour.
The minimal default is the File System Shield and the Web Shield, see attached what I have for my Avast minimal-installation-plus Avast Free installation.