Formal:
* This is highly ingenious.
* This is remarkably clever.
* This demonstrates great ingenuity.
* This is a very astute observation.
* This is a brilliant solution.
Informal:
* That's really smart!
* Wow, that's clever!
* You're a genius!
* That's a great idea!
* That's so resourceful.
More specific:
* This is a very creative solution.
* This is a very clever way to approach the problem.
* I love how you thought outside the box.
* This is a very insightful observation.
* This is a very well-thought-out plan.
The best way to say it will depend on the context and your relationship with the person you are talking to.