Avira is currently my free antivirus choice. Very light, effective. I like the interface too, no problems there. You have the cloud and the option to review files to be submitted (a must, which 360TS also has).
avast! is good, but I found a bug, very similar to one in 360TS, in which the real-time protection is a little faulty (summing it up: download "eicar.com", no detection; copy-paste it, "eicar.com - Copy" gets detected).
So I'm actually using both. Avira is on in real-time, and avast! has no real-time shield installed (plus its self-defense is disabled, mutual exclusions are set, etc), so I can use avast! as a second opinion scanner that is always up-to-date, unlike most real on-demand tools where you have to wait for updates to finish. avast! uses nearly no memory and consumes very little CPU in this mode.
360TS, for comparison, is always busy; no matter how you set it, even turning off all HIPS features and all real-time protection, "QHActiveDefense.exe" is always busy consuming CPU cycles. This product is still very buggy in my opinion, not recommended.
On the paid sphere, ESET, Emsisoft, Kaspersky, Symantec are some marks I would consider to buy.