171

This number is a composite.

10¹⁷¹ - 171 is prime. [Broadhurst and Titanix]

π(1)!*π(2)!*π(3)!*...*π(171)! - 1 is a 3857-digit prime. [Firoozbakht]

(There are 5 curios for this number that have not yet been approved by an editor.)