Asking for "the best" is subjective. I've never used the following but recent HN post regarding wireframe apps for Apple iPad was:

http://news.ycombinator.com/item?id=2090046

http://www.imockups.com/

Using the search link on HN with the term <wireframe> should provide plenty of hits. I think there's a YC funded company that deals with wireframe software, but I'm not certain if this is correct?