The smartest dogs are apparently poodles and Border collies, but they're smart in different ways, I think. There are large poodles, but their fur is a lot of work to take care of. Border collies need work or they can be problematic due to suffering from boredom.
I have a huge dog, a mix of St. Bernard and Burmese Mountain dog. My mother called her a pony when she saw her. She's a very big dog. Makes very big poops. She's a bit of a stupid but very loyal dog. I find some dogs get their smarts only after two or three years. Before that, they're just entirely attention deficit disorder. She's so big I can't wash her, not that she will hold still if I do. It took me five years to get her to hold still for brushing. She was just too happy and attention deficit before that. When the dog whisperer met a dog with the same character as mine, he got tired in half an hour, lol.