The top 3 thrown around are generally always Avast! anti-virus, Avira and AVG Free. I'll try to give you a little break-down of where each has its' strengths and weaknesses.
http://free.avg.com/ -
AVG: The current release is version 8.5, and ever since version 8.0+ people have complained that this particular program has become bloated and sluggish compared to its old performance. I can't speak to this issue as I've never tried it at length, but outside of those problems AVG was a fan-favorite for a very long time for dependable performance.
http://www.avast.com/eng/download-avast-home.html -
Avast!: Avast I feel for most people is the best choice in general unless a light-weight footprint is important for you, in a case such as an old computer, laptop or netbook. A downside to this program is that while scans are very thorough, they take a very long time as well. I used it for a very long time and it's been touted as one of the best, but over time I found myself having to turn it off during gaming or anything processor-intensive. Still, very solid and I still choose this over others on my desktop PCs for myself and family/friends.
http://www.free-av.com/ -
Avira: For Avira, I'd label it as the happy medium between performance, speed of scan and light-weight operation. This is what I ended up using after needing to change from Avast and have been happy so far. Scan time length is manageable and memory footprint is unnoticeable for me. I can't speak to virus detection yet, but many other reviews exist on the internet for this purpose anyway, and the reviews are generally very good. The one downside to this program is it pops up a huge ad each time it updates, but a quick google search will show you how to disable this.
Good luck. D;