After trying all the others, I like the cloud-based AVs. And, the best free cloud-based AV is now Panda:http://www.cloudantivirus.com/en/
However, in my opinion, the top spot for the very best antivirus program will be soon replaced by EsetNod32 v.5.0, also cloud-based, and expected to be released within a couple of months. I am currently a Beta 5.0 participant, but the Beta trial has been closed to new participants. You could try the v4.0 for 30 days free, however. And, if you decided to purchase it, whenever the cloud-based version is released, you could update for no additional charge. The site to download is:http://www.eset.com/us/download/free-trial
Here are the test results for Eset:http://www.eset.com/us/home/compare-eset-to-competition/speed
Click on each hyperlink: Superior detection, fast performance, awards and recognitions, and certifications. You will see how it compares with some other highly-rated antivirus programs.
That is just my opinion. You can try different programs, if you like, to see for yourself which is better for your computer. Each computer's configuration sometimes determines what AV works best for it. There are several free trials also, including Betas, that you could download and evaluate.