Christmas time, can't beat the tree, lights & decs! 2nd best time (close first) is right after Labor Day w/e - kids are back in school, parents are at work, the park can be truly empty some days....nice for walking around.
Christmas Time! especially if it is raining, the christmas lights look amazing reflecting off main street. also, I like going in January, crowds are usually small and it can get nice and chilly (fun to bundle up, grab a coffe and decide which ride to go on)
I love the parks in mid-late Autumn. There's nothing better than an early evening ride on the Mark Twain with a cool breeze and a beautiful sunset. The leaves have begun changing their colors, the park is relatively quiet, and everything can be enjoyed at a ho-hum pace.
never had many choices just have to wait for the opportunity to arrise and then pounce otherwise it could be who knows how many more years (well not so much now that I'm older but not being in California really cuts down on ones ability to have a favorate time)
I do remeber one trip around november I think it was the week after thanksgiving when most of the schools had started again (and pre holiday layovers) I loved it because the lines were so much shorter the park was less congested and I got to walk around and do everything I wanted (I also had this experience going last summer though)
In October/November and January-early March. These are the times of year when there are the fewest crowds, and it's great to just go to the park for an hour or two after class and be able to walk onto two or three rides in that time.