I like Balsamiq, although I prefer Axure (wish they made a mac version) simply because I can create a functional prototype and auto-generate functional specs.
The best ones I have used are LucidSpec (http://elegancetech.com) and AxureRP-Pro (http://www.axure.com), but Balsamiq wins for basic conceptions simply because it is web-based and it is easy to collaborate with others across the globe hassle-free.
I prefer Illustrator for sketching because it is easier to drag certain elements straight to Photoshop once you are done. But I will give this a try anyway...