Over time, I've found the answer to be seasonal or at least heavily dependent on cycles of context and mood.
In vague terms, the best work days are when I start, build, and complete something for myself. It could be a prototype, a one-off program, or a feature on an existing application. It's a milestone of encapsulated accomplishment. This is especially nice when it has a visual component I can point to and share with someone. If it's for my employer, that's ok too, but not as fulfilling.
There are other seasons when success is just attending work, making a small step of progress, and not quitting.