I haven't read through every response but I don't think this has been brought up yet...
What surface are we talking about in this hypothetical situation? A lot of posters who are saying they have broken 15:00 did so on track. The half marathon and marathon would be on the roads. In my opinion, breaking 15 in a road 5k would be much harder than breaking 2:30 in the marathon.
Another aspect is the age of the runner. In college I just missed breaking 15:00. As a post-collegiate, it is hard to imagine breaking this barrier. My training is not as focused and you have to be incredibly disciplined to do the proper speed work on your own. Lastly, where I live there aren't any fast races. How many people live in an area where people regularly blast 14:5x at their local 5ks? Without the proper training partners and competition it's just not possible for a lot of people.
Specifically for performances that occur post-college training:
5K (hardest)
13.1
26.2 (easiest)
Obviously situation is different for many types of runners. Thoughts? Can anyone relate?