I have to, have to say Border Collies. Gonzo is ridiculously smart - not just in learning tricks, which takes a few reps and he has it down forever - but in problem-solving, intuition, memory, and intelligence in general. Since BC's are bred to be able to work a large flock of sheep even when the handler is out of range and cannot instruct them, to me they are not a well-bred BC if they don't have amazing smarts.
I will also admit that in Obedience class, Gonzo won all of the fun matches, against 30+ dogs.... and the 3 other BC's placed right in line. I felt bad, because everyone would shrug and be like, "Leave it to a Border Collie!" if only people knew how hard it is to live with a smart dog sometimes, though! I've met equally intelligent Chihuahua's, Poms, Saint Bernards, Shiba Inus, & Huskies who are all ranked lower. It definitely does depend heavily on the owner & dog, but some breeds (especially herding breeds) are just bred to want to please and to learn very quickly, so of course they have the benefit of decades of breeding.