BlackBerry, by far. Even though the company isn't doing well at the moment, their reputation is based on providing a more secure platform for business and government users. They earned that reputation, in part by quickly addressing known vulnerabilities.
During a recent AMA on Reddit, Tim Strazzere from Lookout Mobile Security also mentioned that Amazon was surprisingly good at pushing out updates quickly and responding to his contact.
Samsung and Motorola would probably be best after this, but I think they face more hurdles due to all the different versions of their devices on all the different carriers.