Well, it's hard to say because there almost is no real slow season anymore. Immediately after school starts is usually good, so September after Labor Day week. January is still pretty low too but expect a lot of refurbs (and avoid any school holiday weeks). April-May can be just fine, but avoid Easter and Memorial Day weeks and any Spring Break times (though those are more like March and very early in April). I went the past couple of years on the first week of May and crowds were very manageable.
Note that next year should bring the opening of Pandora at Animal Kingdom, so the crowds could spike around that time. The only time given for that as of now is "2017" so there is no way to plan around this at the moment.