Best you pay for or best which is free? Personally I use and recommend either AVG or Avast! which are both good, and both free.
I use NOD32 on two systems (pay) and Avira on the rest (free). AVG and Avast are good too, though I've had issues with Avast hammering the disk for no good reason.
Free:- AVG or Avast or Microsoft Security Essentials (Beta) Not Free:- Kaspersky IS 2010 or Norton™ Internet Security 2009 All 5 have had good reviews. AVG and Avast obviously being the older free products that people know. The Microsoft one is obviously new but has had some really good reviews. Kaspersky is a long time favorite of a lot of people. Norton had a real bad rep for bloat but lately has really trimmed itself down.
NB This beta is available only to customers in the United States, Israel (English only), People's Republic of China (Simplified Chinese only) and Brazil (Brazilian Portuguese only).
Late reply but...You can download Microsoft Security Esentials from that link and it works fine in the UK.