It depends on the user. If you are a developer, Firefox is the best because it has really cool add-ons which other browsers don't have (e.g; FireFTP) but FF consumes a lot of memory.
Chrome is swift & light - if you are someone who only wishes to browse.
Opera is nice too. It has a very good community but I don't like how it's add-ons are not well integrated.
Flock is a browser for those who are into social networking. It brings everything together.
IE has improved a lot now. You might like it.
I'm using Chrome.