Tabbies. Hands-down. I’ve got two, and they are constantly amazing me with their intelligence. Of course that makes them more work to care for, having to keep ahead of their wits and all. 
Sorry - my tabby’s dumber than Paris Hilton on pot. I think she’s brain-damaged, though, so it might not count.
My long-haired black cat, however…damn, that cat is smart. We think he’s part Turkish Angora - he has the bone structure. He’s one of those cats where you would swear he knows exactly what you’re saying to him. He is smart, smart, smart. I love him dearly, but I’m almost afraid he’s going to outwit us one day. I keep thinking I’m going to come home and find ElzaHub tied up in the kitchen while the cat’s ordering pizza.
(His nickname is Bucky, from Get Fuzzy…).
E.
Some people insist on playing with fire:
Funny that you should mention Leslie Fish. She wrote a great filksong about this very topic. 
(Link goes to lyrics, not a sound file)
I once trained a red tabby to ring the doorbell when he wanted in. However, the smartest ones are hep cats, man, hep cats. Can you dig it?
Dude, I was SO going to mention Leslie Fish. ;> I love her music.