Hands Down Kaspersky has the best heuristics scanner essentially even if a virus, malware, or spyware isnt in the definition database because its new if the file behaves like a known virus it will be quarantined . On the hour updates 24 a day.
Kaspersky Full version is what I use, but there Basic Antivirus/Antispyware is just as good for virus, spyware, and malware. The more expensive versions just have additional tools i like.
Also Webroot Antivirus/Spyware is still one of my favorites. Probably in the top 3 for removing bad content thats all ready on the computer before you install something. Its not a resource hog either.
One thing if you download lots of things off the internet you will here alot of people talking about Nod32 the program is just ok and i mean that. They purposely let past certain programs like Cracks to warez and alot of dumb people think that those files are safe, and all the other big antivirus/antispyware programs have it wrong. They call them false positives. Well if you have a firewall, lol run one of those cracks that is blocked by other programs, and not by Nod32 and monitor your connections. And you shall see it isnt always a clean program.