Subtlest

adjective

1. Hard to grasp; not obvious or easily understood; barely noticeable.

   "The difference is subtle, but you can hear it if you listen carefully."

2. (of a thing) Cleverly contrived.
3. (of a person or animal) Cunning, skillful.
4. Insidious.
5. Tenuous; rarefied; of low density or thin consistency.