Worst experience: Learning how data is stored in segments in a middleware application called PMS on mainframe and how to manipulate that data.

Best Experience: Building a app that lets you pull down any set of segment data from mainframe and figuring out a way to automatically annotate the data so you could just hover over it and you know what the data is exactly. This way I didn't have to constantly refer back to a reference manual to see what a field name is in a segment, or having to go talk to a mainframe developer to go look at their code. Btw, did I mention I made it searchable by field name??