I went with the 1940s, since nothing beats Looney Tunes, which was at its peak in that decade. Also, Popeye was still going strong, Tom and Jerry emerged and Disney cartoons were also strong. In the same way that the 1930s were the golden age of Hollywood, the 1940s were kind of the golden age of animation (largely because of animated films, but cartoons were at their peak, as well).
Modern decades have far greater quantity of cartoons, and with that, naturally, comes some quality, but nearly everything in the 40s was of high quality. They were all made for the big screen, after all. Also, for that reason, they're for all ages and, consequently, timeless, unlike most cartoons since the late 70s that target specific age ranges and lose much of their appeal once you grow out of them.