BT's
We have a Boston named Simon, we got him when Hugo was 9 mths old. My hubby wanted a smaller dog, and we found a good breeder for Bostons. Just like any breed, the temperament really depends on the individual puppy. Mine is the poster child for "little man" syndrome. He marks when we walk, and he will try to pick fights and bully other dogs. We tried to foster a Boxer and it was 48 hrs of our Boston picking fights with the other dog. We had him neutered early when we noticed his dominance issues, but after talking with other BT owners, some of them just tend to have those 'terrier' tendencies. It's not really 'aggression', they are just, on average, a highly dominant breed. On the upside, if we ever had a break-in, they would have a little black and white dog attached to their ankle. Simon has also dug several attempted tunnels to China in our backyard. (another terrier tendency)
Boston are also (typically) harder to housetrain than other breeds, but from what I hear that can be true of a lot of the smaller dogs. Our Boston, Simon, still will try to poo in our family room if we forget to put up the doggie gate. But then again, it took us 5 months to train our Boxer. (they just don't like to go outside when it's cold out)
On the upside, they are incredibly sweet and sensitive. They are absolutely hillarious to watch, our Boston has about 12 tennis balls hidden under the sofa! They don't have the 'doggie' smell, don't really shed, and are relatively healthy and long lived. Simon is starting to get the one health issue for Bostons: patella subluxation. He was racing down the hardwood stairs and slipped.
If you want the small breed most like a Boxer, I don't think you'll get any closer than a Boston Terrier. They are little sweeties and will crack you up on a regular basis with their little attitudes! I'd make sure you find a good breeder and they can match you with the best puppy. Good Luck!!