Wow I am shocked that no one has mentioned Indigos/Cribos!

Their intellegence and bonding with a keeper is what pushed their wild numbers so low. All the way back to the famouse "Reptile Book" by Ditmars (1908) this intellegence is documented. With Indigos allowed to roam in a house actually following keepers.

I kept (fostered) at Yellow Tail Cribo for a few months and I have to say he over time decided he liked me, would climb on his own accord out of his viv and up my body.

I would also say Black Racers seem to be about as smart as any snake. Ditmars mentions one that would crawl across the floor in the same book mentioned above and take a mouse from a keepers hand. I think it is just most are so mean that keeps most people from studying and keeping them.

The other snake (that I do keep) that seems very smart to me is the False Water Cobra (hydronastes gigas).

The Beauty Snakes (Taiwan, Blue, Etc) seem very aware too.

Yet the smartest again I have to say the Indigo/Cribos. Long as you can deal with the massive ammount of waste they are about the most personality you can get in a snake. Now one that bites as an adult will also require you to visit the doc for stitches so thats the trade off