Well, I´m not an expert, but i say Avast because, even they tree are free, and with lots of options:
AnivirPE (is the red umbrella one?) is horrible to update. Is always like reinstalling the hole program, lots of Mbytes, and not everybody has DSL/cable connections.
AVG has not this problem, but its only in english, and they dont answer mails asking for translations.
Avast is in lots of languages, and sure soon will be in much more because they answer when you ask them about that topic.
So, that´s it. As i say, i have no idea about search engines, virus databases and things like that, my criteria is based only in so simple details like the above ones ^_^U