Everybody's experience might be different based on how they use their computer. I have had good experience with AVG although their sigs are not always updated quickly. I've also had good luck with McAfee as far as sigs updated quickly and detection of critters BUT have had performance issues at times. Just my experience.
Just pick one, several have been mentioned on this thread, install it and most importantly: make sure it updates the signatures daily!
Make sure you practice defense-in-depth TOO and not rely solely on AV...
* Harden your operating system by turning off all unneeded services for example
* Keep your op sys updated (turn on Automatic Update for Win boxes)
* Install an anti-spyware program....actually TWO!
* Browse untrusted websites with non-IE web browser that has Active-X and JAVA turned off
...to name a few measures.