On one hand, IE 8 offers an excellent private browsing mode, tab isolation, and great blacklisting of malicious sites. On the other hand, its InPrivate Filter doesn't catch all ads. IE 8 is also the most frequently attacked and exploited browser, though Microsoft puts great effort into patching as quickly as possible.
Despite this, IE 8 for the very inexperienced/naive user is probably the best bet as it blocks more blatantly malicious sites than the rest of the field. Microsoft-sponsored research puts this block rate at 81 percent versus the next closest competitor -- Firefox -- at 27 percent. This may be a bit of an exaggeration, but Microsoft deserves praise for its progress on this front.
Chrome offers good overall protection with tab isolation, a private browsing mode and less vulnerabilities, but it is victim to probably the most ads of any of the browsers. Firefox is a close runner up to IE 8, especially when add-ons are considered. However, it lacks tab isolation. Opera and Apple have both put a fair deal of thought into their security efforts, but they just aren't as strong or focused as those of Microsoft, Mozilla, and Google.