There isn't one, and you shouldn't pick your departure date on this basis. We looked at 643 crossings by month and the differences between them aren't statistically meaningful. November came out rougher and December calmer, but the confidence intervals overlap, so we can't call that a real pattern rather than noise.
The anecdotal version tells the same story. Some of the worst crossings on record happened in February, and some of the best in November.
This is one of the places the industry has a bad habit. You'll see January described as the calm month on plenty of operator and agency pages. Our own data doesn't support it, and we'd rather tell you that than repeat it.
Pick your departure for what you actually want to see, whether that's penguin chicks, whales, or the deep ice. Those things do change by month, and the Drake doesn't.