As you can probably guess from my avatar, my current most favourite cartoon is My Little Pony: Friendship Is Magic; it just makes me extremely happy for no reason at all, which is very uncharacteristic of any other cartoon.. which might even make it my absolute most favourite cartoon of all time, even with the nostalgia factor added in for the others.
In no particular order for the rest,
Tom & Jerry, Looney Tunes, Franklin the turtle, The Wonderful Adventures Of Nils, Seabert, The Adventures of Paddington Bear, The Mask, Dastardly and Muttley in Their Flying Machines, The Wacky Races, Scooby Doo, The Flintstones, The Jetsons, Courage The Cowardly Dog, Ed, Edd & Eddy, Powerpuff Girls, Samurai Jack, The Cramp Twins, Grim Adventures, Time Squad, SOS Croco, Once Upon a Time... Life.
and definitely tons more I've forgotten about