The answer is.... that depends. lol
If you're looking for smaller crowds and shorter lineups then from now until the end of September is good, as is right after Thanksgiving. The best indicator is to check the "My dates are flexible" box when checking out properties and you will see that the prices will start to rise at the end of September, which means there is more demand.
As to where to stay... anywhere in and around Orlando is good. Again... it all depends what you're looking for, how long you're staying, and what you want to pay. If you prepay for your tickets online you will avoid the lineups at the ticket kiosks. If you stay at a Disney resort then you automatically avoid the transportation issues.
We like this property:
11450 Marbella Palm Ct Orlando, FL 32836
It's convenient to all of the Disney parks, water parks and the Boardwalk. Even better, it's much more reasonably priced than the Disney properties. February (kids are out of school for Mardi Gras in Louisiana) or late Fall is a great time to go. We try to avoid the major holidays as the parks get crowded then. It's going to be hot & humid during the summer months, so be prepared for that.
For the smallest crowds go in early December. If you can, wait until Thanksgiving is over but before school is out for winter break. The parks will be decorated for the holidays but the lines will be almost nonexistent as most people will be busy at home preparing for Christmas. I'll admit that I'm a fan of staying at the Disney properties because of the additional perks (transportation, early park entry, etc.). If you plan to only do a Disney related trip, this is the way to go. If you intend to visit other attractions in Orlando (e.g. Universal Studios, Sea World) don't spend the extra money to stay on site. Staying on site also eliminates the need for a car (again unless you want to visit things outside Walt Disney World).
Naturally, there are many Marriott properties in the area. Decide on the focus of your trip and do a little homework. It will help reduce potential hassles.
I found this a good crowd calendar for our visit last year:
We went over Christmas time and yes it was busy but the decorations were beautiful.
The weather at that time of the year was nice also.
If I didn't have children I would visit early December to see all the Christmas decorations but not the crowds at the end of December.
We stayed in the Renaissance SeaWorld which was a good property.