My view is simple, if its connected to the internet, it needs some form of antivirus. There are so many ways to get infected without any user interaction. Remember conficker? A very good read is Worm by the author who wrote Black hawk down - its basically the story about the conficker outbreak and how it evolved. Creating the largest botnet in history to the size of something that could be used as a state weapon.
Machines are so capable nowadays and there's always an option for everyone out there. Check out av-comparatives if you want to see how vendors are currently doing. It literally changes monthly with most of them all doing similarly, So really the best is...depends.
AVG does seem to be a bit slower, but then again the tech support is helpful and have been around for long enough for a decent reputation. Avast is a bit quicker, but has missed some biggies in the past. MSE seems to be my favourite free AV. Just "feels" right in some ways - but i'd happily switch around.
Currently running Kaspersky 2012 cause i get it free with barclays. I'd recommend it and it did well in AV-Comparatives all round.
Used to use Eset back in the day when it was pretty much the ONLY choice for everything. Seemed to drop off in detection rates and got a bit frustrating to use (plus the price tag) but again, this all changes on a monthly basis.