we went last year at MG, and are going again this year. Honestly, with Disney gaining popularity by the minute, I don't know of any time that it isn't busy (school days aside). We've been Thanksgiving, the middle of summer, and find that MG is the best time. Good weather, and good crowds. Remember, you do want some crowds, or the parks can close as early as 6 or 7pm. Sunday night at MK, they are open until 10pm, with EMH going until 1am - can't wait for that!
I agree about the SuperBowl - I think that they definitely added to the crowds last year.