I suppose my 6-1/2"24 ruler is my favorite tool but it's not very exciting. I'm also extremely fond of those little purple snip scissors from Connecting Threads, I probably own 10 of them, I keep them everywhere.
Favorite feature: maybe built-in walking foot; that or the auto thread cutter button.
Wish List: a longarm! And a bigger house so I have a place to PUT it!