From what you are looking for in your future dog I would have to say the choice for you is clear to me, that is unless you plan on leaving him alone in your house all day. Border Collie is what you want, unsurpassed in the attributes you seek especially intelligence wise...

15-20 lb cairn terriers. They hunt constantly for rabbits, snakes, rats, mice, birds, lizzards. Alarm bark for intruders on the property immediately. No dog aggression issues. For protection get a gun. These dogs are smart and notice stuff . I let my dog out front of the house and went out with her. I sat on the porch and she wouldn't leave me, she started sniffing immediately. Turns out a baby rattle snake was hanging on th the brick siding of the house in the mortar. I grabbed up my dog and found my husband and got him to kill that snake. She is very loyal. Male dogs wander off more than the females. Males bite more too. The female only bit me the day she gave birth to four puppies. She is worth every penny. Ancient Scottish dog breed. Bred to hunt the Cairn rock piles. Irish wolf hounds were used to hunt the deer and for protection.

He's amazing!

I have only owned shepherds. Several males and a female. I have friends that have Rots, Dobs, etc. so I have been exposed to a lot of other breeds. shepherds are wonderful dogs both for working and companions. Like most dogs, you need to train them early and they need a lot of mental stimulation and physical activity.

They take to commands very early and are very eager to please. The ones I have had are very food(treat) motivated.

I haven't seen any dog aggression, but they can be people aggressive (mostly leash aggression) but it seems to be based on curiosity rather than fear (i.e. they wag their tails while barking).

They bond with the family like crazy, and actually know when it is the kids bed time and herd them into bed.

Our friends that come over regularly are met with great abandon and they absolutely remember who they like and who they sort of tolerate.

Shedding is an issue, and I try to brush ours regularly so if that isn't something you want to do (or you don't mind the swiffer or vacuuming) then that may be an issue.

I love the breed, they are incredibly soulful and loyal.

Boxer.
The best.
A loyal, tough and intelligent dog that will protect your home and children.