Yes, GMAT in this month, is the toughest so far. The question bank for this month for both quant and verbal is really tough.
ETS is usually in a generous mood in December, as most of their staff should be on leave, etc.
The month of December usually has easier if not the easiest test.
Experience it for yourself, give the test now and then in December, you will know, what I mean. Bonus points in easy questions come in December.
How would you know that this month is "toughest of all"? How you taken the test every month?
In any case, IMO your conclusion about December is absurd. The question bank has been prepared long in advance and there is no sensible purpose for making the test easier just because ETS presumably is short-staffed that month. In case you didn't notice, the scores are calculated by computer.
Second, whether the tests differ in difficulty or not (there is probably some variation in difficulty from month to month, but it is certainly not planned, not predictable) doesn't matter because the scores are NORMED. This means that the scores are adjusted for the differences in difficulty level of the test. A relatively easy test will be relatively easy for EVERYONE, hence the scores will be adjusted to reflect this and the ensuing percentage score will correlate well to a test of different level. In fact, the more difficult the test, IMO, the more accurate the scores because, in a test with only 81 question of which only a portion are "real", you need more difficult test to differentiate at the upper end.
If I were you, I would not worry about the difficulty or lack thereof of any particular test. If you get a difficult test, it will be difficult for EVERYONE. Trying to outsmart the system only takes valuable energy away that you could use to study whats really important.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993