It all depends on your priorities.
If you want to go when it is warm, you should go May-Sept.
If you want to go when there are more moderate temperatures, then March, April or October (and maybe May) should be considered.
If you want to go when the crowds are low (and possibly miss some rides and shows that are closed) then you want January, February, September or October.
If you want to have the most number of rides open and shows running daily (even though the crowds are higher), then you want late March, early April, June, July, August, late November or December.