Kaspersky Internet security 2009. Short of that, I think Norton IS 09 is a close second. Both have great protection imo. I like them because they slow down my pc the least, and I see the fewest amount of issues with them.
Mcafee: Slow's you down and can seriously screw up an internet connection. Protection is terrible
Trend:Gray circle of infinite loading... (Sometimes Trend can break in a way that will cause it to eternally load at boot, and you have to un-install and reinstall to get it going.) And I don't think all that much of the protection.
Webroot: Biggest problem I have with it is the interface. Protection is meh
CAS: Pain in the ass to remove, and can really screw up a computer. Protection is good, but when it breaks, doesn't seem worth it to me.
All the other nortons: Take forever to do anything, slow down your pc a lot, and can stop IE from working... which isn't necessarily a bad thing... good way to start someone on Firefox. Protection is decent.
AVG: Don't think this has the best protection, and updates every once in a while are a pain, but other than that, ok.
Avast: Don't see much of it, so it must be pretty good, but I still like my Kaspersky