Best tool for design scenarios?

I was wondering if there is any favorite or nice tool for design scenarios.
Which tools are your favorite for designing levels?
- note: changing size and positions by code (try and error) is not a good solution

I would like to start a new game project, a platform game, but I would like to let the designer have complete control over the scenarios, (and avoid as much as possible the need of a developer)

I need something that can visualize and order:

  • (LONG scenarios)
  • NOT tiled.

extras:

  • maybe 2 or + layers of deep
  • if its possible to handle distorted sprites

Something like this would be AWESOME