I've had Anacondas, Balls, Corns, Kings, Rats, BCI's. I've also had to house a retic for 6 months.
I can say hands down out of all the snakes I wouldn't necessarily say smarter, but the Anaconda and Retics were the most non-frightful which some would say is more intelligent because they interact more and settle down when comfortable. Balls generally jerk their head back when you go in their tank/tub, though I'm sure they have exceptions to this rule.
Of all the snakes I've dealt with I'd probably put my Rufous Beaked higher than any of them for the simple reason that if my hand goes in the tank they normally retreat their head but still watch. If I go near them and just gently tap the substrate then 2 of the 3 will come to my hand.
My littlest one will come up my hand when I'm misting the tank or if my hand just approaches the tank normally. I can't say this for any of my balls, though my Sable does like to come out when I open the tub if my hand goes nearby he jerks back. Of all snakes intelligence wise and me having little to no experience with them I would probably say Cobras or Black Mambas would be the highest on that list.