Why not Gunung Mulu National Park or Tsingy de Bemaraha (seems quite iconic)?
I was being stingy and limiting my choices! Gunung Mulu would also be a great example of a karst cave. The karst pinnacles are also well known, and something I'd like to see, so I certainly wouldn't mind if it makes the cut! I'm not sure Tsingy de Bemaraha offers anything that Gunung Mulu doesn't have; I'd be more inclined to recognize Madagascar for its fauna with Rainforests of the Atsinanana.
Now that Lena Pillars is in this category, I'd also submit that Lena Pillars would be a unique example of a frost-derived karst. The pillars have looked impressive in all the pictures I've seen.