The WAHC! I have helped NPSF with the WAHC for coming on 5 years. Every year we outdo ourselves from before. The site is a fully-functioning, custom event registration experience. In 2014, it was simply a brochure site with a link to RegOnline. I am proud of the fact that we go above and beyond to make the site beautiful every year. In 2018 we introduced WAHCity.
Difficulties: Nothing serious, I love working on this site because it is a break from my normal routine. We do everything from the site instead of sending users to other places; they checkout here, view sessions, download files from speakers; I guess it works in a similar fashion to the DrupalCon website (except I like our WAHCity pages better!).
Here are some examples of our newest product; WAHCity.