The best small bookshelf speaker you ever heard?

First I need to insert the word owned for heard in your question as I don't consider auditioning speakers for short periods worth all that much.
After establishing that; Revel M-20.

always liked the Proac Response 2

Renaissance Piccolo

I remembered contributing to a similar thread here a couple years ago.

Tidal Amea (still awaiting an opportunity to hear the Amea Diacera which is mor than likely even better).
http://www.tidal-audio.de/english/amea1.htm

In the very compact category (12x6x9), Kiso HB1 by guitar and violin builder Takamine leaps to my mind.
http://www.kisoacoustic.com/introduction.html

Always provided, of course, one does not actually put any of these gems on bookshelves, which probably goes for all or most of the previously mentioned as well!