Avira free doubtlessly for me but depends on what you are looking for, how familiar are you with the program, menus, settings, etc. You must check also if the AV works with your firewall without hiccups. Eventually you must feel comfortable and safe with the chosen solution. Take a look here if you need more opinions:
AVG has better real-time protection but due to number errors and BSOD's caused by it on different machine i would suggest avira.
avg is simply too buggy.