I think the best time varies from family to family. For me it would be when the child is old enough to walk around the parks, don't need a nap and is fully potty trained.
My son was 9 when we took our first trip. We were both in awe(it was also my first trip). He wouldn't ride some of the big rides, but we still had a blast.
We returned 2 ½ years later with my sister's family. My niece had just turned 5 and my nephew was 6 ½. That trip was also a success. We didn't have to deal with naps, strollers or diapers.
My niece was only turned away from 3 rides due to height(Mission Space, Space Mountain, and the crazy hurl ride in AK

). The character's were a huge hit. ( NOT the

sad2
Our trip was during spring break. Both kids were able to walk(or should I say run) around the parks from open to close for 7 days straight. They never needed a stroller.
My sister and BIL have had two more kids since that trip. (My niece is 2 ½ and my nephew is 11 months.) I asked last summer if they wanted to go down with my son and I next spring. They both said heck no, they're not returning until the baby is 5.