this is an almost impossible thread to answer, its all about personal preference.
Internet Explorer has started to get slated, this is more due to firefox hype than anything else and it is in no way as bad as everyone makes out, with automatic updating it can be as safe as any. I currently use IE7 beta and its been outstanding.
Firefox has many plus points and has a huge fan base following now but they shot themselves in the foot by declaring so brazenly how "secure" they are. It was an open invitation to hackers to attack it in everyway possible. Updates have helped this but its still prime target for everyone.
Opera on paper is the greatest browser out there, faster, safer, secure and has had features for years that others are only just starting to implement (tabbed browsing etc) but it can have stability issues and isnt supported as well.
So its all down to what you feel is the better and easier to use yourself. each one is going to be mroe than secure and safe enough for most every user. All are going to meet the needs of people, but some epople prefer the features of one over the other