I can tell you right off that it's a great question, but 20 different people will give you 20 different answers. I'll make that 19, because I won't even recommend one.
Having said that, there are two points. One is DO NOT UNDER ANY CIRCUMSTANCES EVER BUY NORTON OR MCAFEE.
The other is that it depends on whether you are purchasing commercial-grade networked AV (basically runs on one computer and can monitor the others -- virus definitions are downloaded by it, and it sends them to the clients) or home versions (which are still good).
Also, I am not a fan of Internet Suites that combine everything. I would recommend to start with a good AV program and then scan for other malware with a free program like...damn....I almost recommended one. Well, since it's not a true AV program, Malwarebytes'Anti-Malware is hard to beat.
