If you're looking for something free, I'd recommend Avast. If you're running it on default settings, though, then the update notices can be pretty annoying (since it apparently updates its virus database anytime any one of its users detects a virus, and does it all without you needing to give it permission to run larger updates that add a ton of stuff at once like you'd normally expect, which is kind of clever) but you can easily just turn those notifications off and won't have to worry about it bothering you anymore. Then it can do its job in the background and, once again, won't bug you about needing to install updates. Worked pretty well for me when I had a PC.
Currently, I have Sophos, which I really only installed because my school recommended it. Currently all seems to be well, but, eh, Macs aren't at quite as much risk, anyway -- so I'm really not sure if I can thank Sophos for the lack of viruses or not, since I haven't tried any other antivirus software on my Mac.