I'm sure Disney is awesome for all ages, but if you couldn't afford to go every year or every few years...what age do you think kids get the most out of it?

We want to take our girls to Disney some day but I think it would only be able to be a one time thing. I would want to get the most out of it so I'm wondering which age is best. My girls are 19 months apart so maybe 4 and 6? Younger? Older?