At the moment it is IE8. I used IE9, liked it, but I dislike installing anything thats beta (Just like my computers running with same versions of drivers, software etc, otherwise i get confused and annoyed!). Stuck with IE since IE7, ive never had a problem. Also im too lazy to update it, just let windows update do it.
Beta's aside, i think chrome is possibly best all round atm from what ive read. Although, once IE9 is out, it'll be the best if the beta is anything to go by - it's scared mozilla into pushing back their release date at least.
EDIT:
Vista forums web browser is cool, i just wish it was finished!