This year was my 7th PyCon, I've been to every one since 2008. The most consistent trend in my attendance has been that over the years, I've gone to fewer and fewer talks, and spent more and more time volunteering. As a result, I can't tell you what the best talks to watch are (though I recommend watching absolutely anything that sounds interesting online). Nonetheless, I wanted to write down the two defining events at PyCon for me.
The first is the swag bag stuffing. This event occurs every year on the Thursday before the conference. Dozens of companies provide swag for PyCon to distribute to our attendees, and we need to get it into over 2,000 bags. This is one of the things that defines the Python community for me. By all rights, this should be terribly boring and monotonous work, but PyCon has turned it into an incredibly fun, and social event. Starting at 11AM, half a dozen of us unpacked box after box from our sponsors, and set the area up. At 3PM, over one hundred volunteers showed up to help us operate the human assembly line, and in less than two and a half hours, we'd filled the bags.
The second event I wanted to highlight was an open space session, on Composition. For over two hours, a few dozen people discussed the problems with inheritance, the need for explicit interface definition, what the most idiomatic ways to use decorators are, and other big picture software engineering topics. We talked about design mistakes we'd all made in our past, and discussed refactoring strategies to improve code.
These events are what make PyCon special for me: community, and technical excellence, in one place.
PS: You should totally watch my two talks. One is about pickle and the other is about performance.