For me, a constellation is first and foremost the pattern made by its brightest stars. I do also appreciate the deep-sky treasures contained within, but they're more like frosting than cake.
By that standard, I'd have to say that Orion is in a class of its own. Those seven bright stars really hang together as a recognizable group; it's hard to imagine any culture that wouldn't see them as a whole, or as part of a whole. And they really do remind me of a human being; I think I would have come to that interpretation even if I hadn't known about it before. I can't say that about any other constellation, though Scorpius comes pretty close. Leo makes a perfectly plausible lion, but would be an equally plausible pony or dog.
Orion's Belt is also unique; it is by far the most prominent star triplet in the sky. Think about it. Nowhere else in the sky are three stars that bright in such close proximity -- not even close. And they're in an almost straight, evenly spaced line, to boot.
And then, Orion does have one or two decent deep-sky objects. 
Carina has much better deep-sky objects, but its star pattern doesn't even come close to Orion's.
Scorpius rates right behind Orion in terms of star pattern, and extremely high up in the DSO department.
The Teapot has a fine star pattern; I'm not sure I'd say the same for Sagittarius overall. And yeah, it's got one or two decent deep-sky objects as well.
Oh, and then there's Cassiopeia. And Perseus. And Cygnus. And .... oh never mind.