Okay so after a few days of thinking I think I'm sure about what I'm about to write :
Best : Discovering how to use streams while making a service that should extract a tar.gz, extract the tar.gz within it, filter the extracted files and correct some of them, then compress each folder as tar.gz and compress all the archives as .zip. The amazing thing for me is that with streams I could do all the operations in just two passes, maybe one if I had more time, saving disk writing time.
Worst : upgrading a bunch of legacy Access 97 apps and its VBA code to Access 2013