Norton 360. I think it was on discount when I bought it with the computer, but Which rated it quite highly. I've just renewed mine and it was a bit of a rip off renewing by clicking on the window. I paid £65 when it only cost £37 to renew last year via a website I'd found (76% inflation). I couldn't find the same site again this year. I found numerous sellers offering it for about £30 on Amazon, but I'm not sure how to tell whether you're buying the latest version, or even whether it just downloads the latest info anyway when it's installed.
It's a bit quirky at times, but I don't have experience of anything else to compare it with. Occasionally it will go from green to an amber warning and say that I need to run a scan, which makes me wonder why don't you just get on with it, then. Other times it will show that the last scan it performed was autonomous, without my input. 
Sometimes the Google search page has Norton logos by the side of each hit to show whether the site is safe, but then on other occasions it doesn't. 
A lot of people rate the free ones, but I'm never quite sure how comprehensive they are, and I don't suppose there's anyone at the other end of the phone if anything goes wrong either. When I first bought Norton it wouldn't install, it just kept saying there's some sort of problem and "click here to fix" but I just went round in circles for a whole morning. Eventually, I rang the call centre and someone did it for me.