"I'm defining 'power' as a non-specific mix of expressiveness, terseness, maintainability, readability and flexibility"

There might be room for some unspecified mix of these qualities on an argument over the "bestness" of a language.

But I'll quibble here and mention that power is conventionally defined as ONLY expressiveness/terseness with no consideration of maintainability.

In the J programming language:

    quicksort=: (($:@(<#[) , (=#[) , $:@(>#[)) ({~ ?@#)) ^: (1<#)

http://en.wikipedia.org/wiki/J_(programming_language)

Now that's raw power... (and may not fit all requirements!)