IMO, crowd wise the week after Thanksgiving Weekend through the Week before Christmas Week are the best. With Christmas Week through New Years Week being the worst Crowd wise.
I agree with disneychrista. We always go the first week of December. The crowds are light, the weather is great, and the decorations are just beautiful. The other benefit is that most of the major attractions have already had maintenance/refurbishments completed in anticipation of the increased attendance during Thanksgiving. Several years ago we went during October and were dismayed to find that three "e-ticket" rides were not operating.
My second choice would be late May. Most kids are still in school so the crowds are much lighter than the summer months but most of the seasonal entertainment has already started.
Oh what the heck...go anytime you want. It's Disneyland!!!