I can’t decide on a single one… so I have to share three 
“Ser un bicho raro”.
- Literally: To be a strange creature. Meaning being a weirdo. It was one of the first expressions I learned when I moved to Buenos Aires, because the whole city was covered with an advertisement that said: “No soy un bicho raro!”
"Quién fue a Sevilla, perdió su silla." - Literally: Who went to Sevilla, lost his seat. Meaning: You snooze, you lose. I have friends who are from Sevilla, and every time someone mentions Sevilla, I have to think about this saying. I love idioms that rhyme!
"Más chulo que un ocho" - Literally: Cooler than an eight.
Meaning: to be too cool, sometimes arrogant. I can’t help but wonder: why the eight? If you know the answer, please write a post! 