The ball python I was referring to in my previous post can be considered one of the exceptions. She's not head shy and I think this more confident personality allows for a greater opportunity to observe this interactive behavior. That's probably why people will say the large, formidable constrictors such as retics and anacondas are more 'intelligent' because they aren't as shy. (I think, though, instead of referring to intelligence, it should be referred to as being more interactive and personalized, although snakes were proven in an experience to recognize locations and develop "mental mapping." I wouldn't be surprised if they could learn to recognize other things as well, and be aware of differences or changes.)
King cobras, hands down. These things will actually build a nest for their eggs.