I have really bad feet and I do a lot of walking during the day and so far I have found that Louis Vuitton loafers to be the most comfortable. They are really pricey but i feel they are worth the money because you will get a lot of wear out of them. Also my mom swears by the miu miu jeweled flats
This questions has been posted many times. People normally suggest
Chanel the classic ballet, the cambon flats and the ones with the elastic.
Gucci Bamboo Ballerina.
London Sole/French Sole
Here are some threads that have been posted and do a search to find tons more!