Border Collie.
My parents owned one (which I took care of plenty of times) and they have a mutt now that looks and acts 90% Border Collie. I absolutely adore them.
They are
incredible dogs. They are constatly rated as the most intelligent breed there is and they ace agility tournaments, to the point that some minor competitions ban them so other breeds have a chance of success. I can only describe them as a dog with
the brain of Stephen Hawking rolled into
Usain Bolt's body. There were times when I looked into my old dog's eyes and I thought "that bitch could do my taxes".
Smarter than most small kids (fuck, I'd say they are smarter than many adults), lovely, caring companions and with an utmost desire to exercise. Alas, those dogs have been breed to to work, which means that they need something to keep them occupied or they will become engines of destruction. I kid you not.
Let to their own devices, bored Border Collies will chomp away hoses, dog houses, plants and wreck havok in ways you couldn't imagine. Which can be problematic, given that those fuckers are extremely agile and
can easily jump through a window, unlock a door and climb to the oddest places.
They are also very motion sensitive and have a huge drive, which means they tend to chase cars and
herd little kids as if they were sheep. Which is hilarious, until you let them slip for one second and try to playfully herd a bunch of terrified kids at the park, which may or may not get nipped at their ankles.
Still, they are incredibly lovely, not agressive (despite their hardwired herding instinct), fastidious and their intelligence makes them the most constantly amusing breed that I know. But I live in an apartment now, and although there are some calmer, chiller Border Collies that can live in smaller places, a house with a huge garden or yard (or better yet, an actual farm) is the best place for them.
Also, the shedding. Oh God, the shedding.
Right now they are not for me, but I know I'll be getting a couple of them if I ever move to the country.