I've always read that mambas and cobras were the most intelligent. I've seen nothing in any of the colubrids or boids I've seen or worked with to lead me to believe they have any significant intelligence.
Lizards I think are another story. Tegus and some monitors seem to show fairly complex behavior IMO... You'd need studies to prove it, but I think some of them can tell individual humans apart, which requires a bit of intelligence.
1 adult bull snake: "Dozer"; 1.1 juvenile bull snakes: Oscar and Phoebe; 3 baby red-sided garters; 1.1 macklot's pythons