I honestly don't believe for one second that one breed is smarter than another. I've been around dogs my whole life (havn't lived one day without them) and I must say that I have met both "smart" and "dumb" dogs from different breeds.
I think it depends on these things:
Your Personality.
The Dogs Personality.
Training Method being used.
Your knowledge of doggy behavior.
Your consistancy!!
Research different breeds, and decide which one matches you the most. If you go to train a dog who is extremely hyper, thinks quickly, and over thinks alot in general, but you are the kind of person who is lazy and pretty inconsistant with what words/signals you use to get your dog to listen-- then you will have a problem!
Make sense? I tried to explain it how I see it, sorry if it came across as a bit out there or confusing-- I keep getting side tracked!!