I don't think you can really measure a snake's intelligence by how attentive it is. It denotes awarenes, curiosity, yes, but I don't really see how a snake that watches you is smarter than a snake that spends most of its time in his hide - that, if anything, has to do with "personnality".
I've always been a firm believer that you can't measure the intelligence of snakes. Not to say that it doesn't exist, but they aren't "smart" the way we think of "smart". A dog, cat, horse (any mammal, really) can learn basic or complicated tricks, the teaching of which is based on wild behaviour. Snakes don't really have social groups, they have no reason to act any particular way other than to survive and breed.
So I guess it all boils down to your definition of "intelligent". If curiosity = intelligence, then the aforementioned snakes are probably the most "intelligent". I'm not sure how you'd even go about measuring any other type of intelligence in a snake :S Any ideas? lol