Well, when talking about RAM consuming, it's imperative to know what machine you are using. I think the one that consumes the most is BitDefender, followed by Kaspersky. I'd choose Avira if i were looking for lightness, Kaspersky or BitDefender if looking for protection.
Avira maintains the optimization even though they add some processes, for sure those possible additional bootup are already improved from numerous versions.
Meanwhile their free version of Bitdefender is way simple so the resources isn't sacrifice much. On the other hand Kasperksy may have different perspective operation through system to system so mix reaction about lightest.
At the end of day, 'lightest' of a product is a least priority on majority of users where prevention and detection remains to be the number one need.
Is this for AV version or for IS versions?
In my experience, Avira has been the least resource consumption solution, out of the 3 listed.
Kaspersky has a busier CPU when the scans run in the background, however BitDefender somehow makes the system slightly slower compared to Kaspersky. Bare in mind that on an High End system, the resource usage is not that noticeable.
If your system is not High End and you are focusing on resources, then stick to Avira.
If you want more features and protections, then thats a different story, but original post should be edited to reflect such request.
Avira is lightest based on my tests, using between 50-60MB at idle. Kaspersky is next, using 70-100 at idle. Bitdefender as usual uses almost 200MB at idle (as much as McAfee!). As far as subjective "lightness" tests, I sense the least slow down with Avira, followed by Bitdefender, with Kaspersky feeling the most sluggish (which is not bad compared to some others). Personally I won't use Bitdefender products (or Mcafee) simply because they use so much private memory at idle. I'd go with Avira Pro in your case.
Faster(higher is better)
Detection(higher is better)
Fp(higher is better)
Proactive Defense(higher is better)
I go with Kaspersky.