Going against the flow here, but I think Apple makes the most sense. It isn't about the technology, it's about the branding. They are not direct competitors so neither company loses anything by partnering. Both would bring a substantial customer base to the partnership. Both consistently rank near the top of the world's most admired companies so the risk of either company taking a hit to their reputations from the other is low.
Having Canon certified iPhone cameras and Apple certified connectivity in cameras would be a marketing plus for both companies. Canon has struggled to replace its once highly lucrative point-and-shoot market and a partnership with Apple would give them invaluable insights to develop products that appeal to the iPhone generation, especially if those products can be directly integrated into the iPhone ecosystem.
And, while it isn't about the technology, the research and development they both invest heavily in is valuable to the other, without being overly duplicative.