I can't pull out a specific year. I did love 1991 as well, because that was one heck of a year. Darkwing Duck especially.
But I go decade by decade myself.
My first favorite decade is the 1940's, when animation became more fine tuned, and we were out of "look what I can do" (i.e. Flowers and Trees) and into "Look how I can use it." A lot of early cartoons from the 30's were experiments in sound and since they just aren't as impressive, they just aren't that great. However, the Fliecher brothers put out their best imput during this decade with their Popeye cartoons, their Betty Boop shorts (with Cab Calloway doing vocals) and various others. They did a lot of good stuff in the 40's, and their Superman cartoons (with the exception of war propaganda cartoons) were ahead of their time, with Lois Lane a very early feminist role model.
The late 50's and early 60's were the time the TV cartoon was perfected, with Hanna Barbera no longer making Tom and Jerry shorts, they turned to making things like Ruff and Ready, and Yogi Bear. And Jay Ward's sadly short lived cartoon empire had begun.
By the mid to late 60's Superhero cartoons dominated television. My personal favorite being Mighty Heroes, a Terrytoons cartoon created and animated by Ralph Bakshi. But some others came out of this short fad.... Scooby Doo and Wacky Races from Hanna Barbera and George of the Jungle from Ward productions. Sadly the last cartoon the company would sell to television (several pilots were made up until the late 70's).
The 70's was a blur of Scooby Doo knockoffs and animated versions of sitcoms. But we had Fat Albert, the wonderful educational series from Bill Cosby's mind (fun fact, Paul Dini got his start writing for the Brown Hornet segments), and Hong Kong Phooey.
The 80's brought a more commercial approach where toylines ruled television. While in pronciple I do not like that idea, a lot of very good cartoons came out of it. And I'd be a hypocrit saying they were bad, since I did love Super Mario cartoons. We also saw the Rise of DIC animation, with Inspector Gadget (one of my absolute favorites), and Disney's Television animation with Ducktales. And did I forget to mention TMNT?
the 90's saw Disney continuing and forming the Disney afternoon, with Darkwing and Talespin. The Simpsons became a household name, and still is. Nickelodeon started a bold movement with the first original cable cartoon series with Doug, Rugrats, and Ren and Stimpy (which would later give rise to creator driven cartoons). Fox started with a bold line up, and becoming the number one network for cartoons on saturday mornings. They also created 3 movements.... Batman the animated series brought a dark, realistic edge not seen in cartoons on children's televisions. Then with the arrise of The Tick which brought about a short lived movement of comedy superheroes (like Freakazoid and Earthworm Jim), and popularising indie comics (also with the Tick, to be fair TMNT was the first cartoon based off an indie comic). Kid's WB started out with a line up so wonderful, I just can't compaire it to the last decade's line up (which got increasingly worse). 1999 saw the movement of animated sitcoms like Family Guy, Futurama, and Dilbert. Only a couple still exist today, and that was due to fan protest.
Lately, while we have some great creative shows about, most of them are toy commercials, like the 80's but none of them are as good.