I always will recommend getting a dog from a pound or animal rescue league before buying, but since you are requesting suggestions for a purebred dog, I think Rat Terriers seem like nice dogs.
Here are some pics:
These are not pics of my dog (I have a mix that we took in when the grandparents died), but instead are pictures I came across online while searching for information on Rat Terriers.
Lastly, there is a neat site that ranks dogs based on their intellegence. It can be viewed
HERE.
Good luck with your decision.