My 2 cents... For the vast majority of the world where legacy (or at least not-programmed-using-super-modern-standards) systems are still in play, clarity is the most important factor in software. What happens when a legacy system is actually returning the number of seconds since epoch versus milliseconds? What if they're returning ticks (looking at you .NET...) because nobody realized ticks are inconsistent across different systems at the time the system was written?

I don't think you can get any more clear than an ISO timestamp (WITH time zone). Unix timestamps are great and efficient, but I personally value the absolute clarity of ISO above all else.