This depends on what you are looking for as far as the best time. Christmas time is one of the most beautiful times of the year. Each resort is decorated for Christmas and the decorations are beautiful. If you go during this time you have to visit the resorts. They have a carousel made out of chocolate, a a very large gingerbread house and other amazing things. Plus, the christmas trees are beautiful as well. Also, the whether is nicer. Usually it will run in the low 70s during the day. If you are near the water it is a little cold and windy, but not too bad. Also, the parades in the park are themed for Christmas and very nice to see. However, if you have never been to Disney, you may want to see the regular parades. The crowds can be bad, but I have gone several times during Christmas and it is a great time of the year to go.
I highly recommend staying away from the time around July 4. During this time almost all of the parks will close due to capacity. So lines are extremely long and it is terrible to try to get around. Also, it is very hard. Combine the large crowds and the heat and it can be misserable. You will not have as much fun if it is this hot and crowded as you would otherwise. however, the fireworks during this time are spectacular and will not dissapoint. But you have to wait a long time in order to see them from a good location.
I usually prefer going after school has started. September/October is a great time. The weather is nice, crowds aren't and big as the other times of the year. If you go in value season you can save yourself some money. Keep in mind, Disney will often do refurbishments during this time because it is not as busy, so some attractions may be closed while you are there. But it easy to find this out ahead of time. Also, if you go in October you can go to Mickey's not so scary halloween. This is a must see if you go during this time. The park is closed early that day and you have to have another ticket to get in. But there is a fraction of the people in the park. There are rides open and you basically walk right on. It is nice to be there when it isn't as busy and it is really neat as well. Plus, if you have kids, it is more kid friendly than the Universal Halloween party.
So you need to think about what you prefer and then take your vacation at that time of year.