I don't know that there are any good months all throughout...one person's experience on the SAME DAY can be different that someone else's...let alone the same month. Important aspects like weather can be luck of the draw.
Here are my experiences since I've been on DISboards.
2011 - Friday after Thanksgiving through Tuesday after Thanksgiving - Pros: Nice weather, medium crowds shifting to low on Sunday evening, all of Christmas decor up. Con: Big Thunder Mountain Railroad and Mark Twain went down for short/unannounced (as far as I know) refurbishments on Monday; short hours on Monday and Tuesday, but those were kind of a pro too because it made the trip more restful!
2010 - Mid October (1.5 days, a Friday and Saturday) Pros: Halloween decor; Halloween party is a possibility; met Duffy for the first time

and also Jack Sparrow. Cons: Felt extremely crowded to me (New Orleans Square turned into gridlock); party was uncomfortably crowded at times; weather was that obnoxious weather where you're not sure if you want a jacket or not.
2010 - Early-to-mid June (right around the time World of Color premiered) - Pros: Nice weather, fairly light crowds, World of Color premiere decor the day of the private premiere, got to see World of Color's first general public performance. Cons: Short hours for Grad Nite one night (works differently now) and super-short hours for DCA due to World of Color premiere. No full entertainment until Friday; it was still "low season" on Wednesday and Thursday. (We arrived Wednesday and left Sunday.)
2009 - Early-to-mid December - Pros: Light crowds all week and weekend; all Christmas decor up; Fantasmic! ran on Wednesday and Thursday and DL stayed open til 11 even though the parks weren't that crowded. (This was likely due to early closures for Cast Member Parties, which aren't even happening this year.) Cons: It rained a LOT, which is why crowds for the weekend were light. Very difficult to find a place to eat and stay dry. One of my most annoying Disney memories comes from this trip (woman saving 2 tables for her party at Hungry Bear restaurant, and refused to allow us to sit there, though those were the ONLY empty tables in the dry area...her party didn't even come down with their food until we were literally on our last fries!).
2009 - Mid-to-late June - Pros: Long hours; was in
Disneyland on my birthday. Cons: Extremely crowded (people were mooing in the herd toward Fantasmic! and Mom saw a little girl get elbowed in the face!). Lines were long. We used RideMax but the park still felt very busy and we were stressed trying to follow it. Weather felt very hot to me. I got sick so it may have been a low fever talking.