While Android can not beat because Google already patch the XSS problem. You can read further about Android in Pwn2Own hacking contest in Android in Pwn2Own Hacking Contest
iOS on the iPhone 4 and BlackBerry Torch beaten by hacker using exploit weaknesses in webkit. Webkit is the rendering engine in iPhone and BlackBerry. The hacker try to penetrate the security hole and slid a malicious program to steal data using webkit problem.
For browser category Chrome 9 and Firefox 3.6 still unbeaten by hacker till now.