What's the repair you're most proud of? I'm not talking about stuff that would put your bike in the "alt" forum. I'm talking about the repair and maintenance that keeps your bike running or makes it run better.
For me, it was overhauling my hubs. I loved knowing that I had taken apart that "black box," cleaned it, and put them back together correctly.
The only thing I ask is that people don't flame someone who lists a "simple" repair. Every experienced mechanic had to change his or her first tire. And, at the time, it probably felt pretty darned good.