Just had a look through my library.
I'm going to nominate this photo as my most significant. Tough Guy 26th January 2013 was one of my biggest achievements I have physically done. It has a sentimental value to me despite the lack of thought behind the photo comp itself. I took an indestructible Fujifilm XP around with me to try get as much as I could, however, trying to keep alive was more of a priority than getting the camera out....
We had hail, mud, sludge, ice, freezing ponds/moats to swim through, electric fences, barbed wire, obstacle courses the size of multi story car parks, diving boards, barrel dunks, and much more to name within the painstaking 8 mile trawl dripping wet reaching closer to hypothermia than anyone could ever anticipate......... But I did it, and have the medal to prove it.
So yeah, I vouch this one to be significant to me.

Tough Guy 26th Januaury 2013 3 by
DanCatt, on Flickr