Answer:
No matter what kind of horse/pony you buy it will be hard work, if you board your horse that work load is a LOT lighter. But out of all the ponies I would say a miniature would be the absolute easiest; The don't need as much space or food and are easier to clean up after. But you can not ride them unless you are three years old (they are about the size of a large dog).
If you are looking for a horse/pony that would be easier to care for, as in they don't get sick as often, then your best bet is a mutt; A Mix of a couple of horse breeds. Although people gravitate to "Good" horse or purebreds for most breeds that doesn't make them good - a lot of the time it just makes them inbred. So get your self a mix of a couple or more sturdy breeds.
i.e. My horse is a Welsh/POA/Quarter Horse. POA's are not the best horse ever, only because they have Appaloosa in them, which a pron to going blind (which my horse is) but in his case it is balanced out by the Welsh's strong legs and the Quarter Horse's powerful hindquarters. Because he is not a purebred he has no health problems other then he is blind, he also has a sensitivity to sugar but that is not from breeding.
There I am done, now my fingers hurt from typing!
Mfire