Not replying to the best part. Just the server load part. WebAdmin Of the commercial products, LxAdmin HIB is the lowest Plesk DirectAdmin cPanel
I'd recommend DirectAdmin (using it on my VPS) and its showing to be very low on resources. Cheap too...highly recommended.
If you are looking for low resource, go for DirectAdmin and you might want to shutdown unnecessary services to conserve ram as well.
i only have chance to use cpanel and hspear. I suggest cpanel. have never heard of directadmin though. And why would people choose directadmin over cpanel?
Out of all I have used (Cpanel,Helm,Plesk,Directadmin,Hostcontroller) I will say Directadmin is the best and next comes Plesk