The most fulfillment would be where it actually gets results. The first hackathon I went to, I made a simple app according to specifications. I was willing to polish up some features after the event and put it into production, but multiple emails to the organization's contact went unanswered.

I almost didn't attend the event again, but the second time, my project went into production and is still going. The organization was manually copying data from their donation site into SalesForce, and I wrote a small script to do it daily. (The donation site has a "connector" for SalesForce, but apparently it didn't work for them)

I'm currently doing something longer term with an organization I found on UN Volunteers. I was expecting to add features according to specifications, but they wanted ideas for how to improve the app. How about more content? "We don't want to add more content." How about multilanguage support? I was thinking I'd be adding one or two languages, and suddenly he has 19 translators lined up. sigh