Comparing the free ones,
Am using Kaspersky after comparing against Bitdefender. In terms of resource use, both are quite similar for low-resource machines. Reviews show these two to be quite on par in terms of detection strength. In terms of interface, I like the extra bit of control that Kaspersky has over Bitdefender (a little oversimplified for my liking). Boils down to user preferences for both of these.
I liked KSC enough to subscribe to KTS for 2y, but the free KSC is more than sufficient.