You'll hear many different opinions on this...it is becoming common for some people to state there is no longer an off-season at WDW.
That being said, I have always liked visiting in September/October, or January/Early February (avoiding holiday weekends such as President's Day). Particularly in Jan/Feb, the weather is milder (some say cold, I refuse to say as much, since Jan/Feb for me is teens and 20s). Also, the hotel rates tend to be cheaper during these times as well (you can get the official "Value" rates at disneyworld.com).
Whenever you decide to go, have a great time on your trip!
"When entering the theater, please move as far to the end of the row as possible. Stopping in the middle is distinctly unpatriotic." -Sam Eagle
"I may have invented these bifocals I'm wearing, but I can assure you they're not rose-colored" -Ben Franklin, The American Adventure
I would also say January to early February. A few years ago we went the first week of February and were able to walk on to most rides. Really is a good time to go; the temperatures areusually in the 50-60's and doesn't rain every day.
"What would you like most to do - what is your dream?" Proud volunteer for the Dream Factory of St. Louis. check out web site: www.stldreamfactory.org
In Feb. last year it was actually a bit on the cold side. Still, that's a minor complaint compared to short wait times for rides and attractions, as well as being able to walk into restaurants without having a reservation and get seated right away.
Trip in September of 2002 was also not very crowded, although it was fairly hot then.
Possibly going there in mid-October this year, so we'll see.
My vote goes for Sept/Oct for the Food & Wine Festival and Mickey's Not So Scary Halloween Party. Those little extras make the parks a lot of fun! While you do have the change of a hurricane and some rain, the weather is a bit nicer - without being too cold or too humid (hopefully.)
But overall, ANY TIME THE KIDS ARE IN SCHOOL is the best time to go.
Crowd wise, the two least visted months of the year are always September and January.
But with the free food offer the past few years, September has picked up.
And the Disney marathon tends to extend holiday crowds into the first few weeks of the year.
But everything considered, I believe the best time of year for a WDW visit is October. You have weather. You have decent park hours. You have Food and Wine Fest at Epcot and MNSSHP at MK. You have the Funai Golf Classic (that local Tiger Woods usually never misses). ... just a lot going on.
After Oct., I also enjoy April (post spring break) and May.