It's hard to pick the greatest, because there were quite a few achievements/learning experiences.
I think my greatest achievement, overall, was finally allowing myself to go with my gut (pun not intended) on dishes even if some people were telling me otherwise. It resulted in some pretty killer Xmas roasts, sides and desserts. I don't need more proof than that to know there must be something right in my instincts.
Yeah I know how corny that must have sounded, I couldn't help but feel a little proud with myself for it though