The most memorable syntactic SD for me was "litotes". It is really interesting when two negations that have negative meaning can joint to give positive evaluation. It describes the object to which it refers not directly, but through the negation of the opposite. I often use "not bad" instead of "good" in my speech but I did not think how can these two words give positive meaning.
I also like to use such syntactic SD like "Question-in-the-narrative" especially when I am proving something to somebody. It also helps to chain the attention of listener or when you want to focus the audience's attention on something specific
"Break-in-the-Narrative" also interesting SD. We often use it in our speech and writing. Mostly I use this SD when I do not have anything to say, avoiding from the answer or when I do not want to call the things their name.