Panda is reputed to be the most sensitive - picks up the highest percentage of known viruses etc.
PC-cillin is almost as sensitive, & is very good for not slowing down your system because it uses minimal resources. Norton is reputed to be good, but not as sensitive, has a good auto-update feature, but uses more resources. McAfee is supposed to be not so sensitive, uses resources like Norton, and has given me lots of trouble when I've tried to use it.
InoculateIt is short on features, medium in the sensitivity department, uses noticeable resources, but is free. The others all cost, but that's what gets you your updates.
My personal preference is PC-cillin, but that's because I dislike resident programs that drag down my system speed and responsiveness. Updates are only semi-automatic, and it costs around $30.