I'd have to say a cat, why because of my brother. He hates cats and he and his wife are the laziest people I've seen. If they can take care of a cat then anyone can. Especially if you keep it inside all you really have to do is feed it and clean it's litter box. If it want love it'll come to you.
Other than that I wouldn't consider any of them easy so much as a labor of love.
I voted for Betta for one simple reason. You don't have to worry about property damage while training. XD
I'm pretty sure someone has had the puppy that chewed up this, the kitten that clawed up that, or the baby small animal or bird that peed or pooped on some type of fabric. Water changes aren't the funnest thing in the world to do, but I don't have to worry about coming home to a wrecked apartment because the fish got loose.
Since you don't have to train a fish in any way, shape, or form it is the easiest pet in the world with turtles coming in a close second, IMO.
I find it interesting that so many people find dogs easy to care for. I wonder if dogs really are easy, or if they are so common as pets that society makes it easy. Like, you'll find dog parks and dog walkers sooner than you'll find snake parks and ball phyton sitters. XD
Out of all the pets I have, the toads are the easiest. By easiest, I mean we spend the least time on upkeep for them. Substrate doesn't have to be changed as often as bedding, and they don't need time out of the aquarium. So, basically, make sure it's clean, change the water, throw in some crickets.