i have a fairly unique perspective. As a kid, i went outside and played in the sun instead of watching cartoons. I started watching cartoons when i got to college, sticking them in one large pool, instead of separate classes determined by decade and nostalgia.
There is a general notion in this country that cartoons are for children, and children are stupid. This means that most cartoons are either simple or just plain idiotic. Simple can be done well, as in the case of Looney toons, where it is predominantly just amusing antics, or it can just be bad. Many 80s cartoons fall into this category, focusing more on action and situation than overall plot or intelligence (although arguably many just cross the stupid line and keep on going). Modern cartoons seem to be dominated by a simple animation style, which makes characters look 2D, like they are cut out of paper (there are definitely exceptions though. I personally hate this style, but often the content is a little bit smarter than the 80s cartoons. OR just stupid to the point where it should die in fire. Usually these are good to tolerable.
The 90's cartoons had better animation than both modern and 80s, and a decent amount of intelligence behind its shows. I tend to enjoy this decade the most, since the content is not hidden behind simple animation. My two overall favorites are the Tick and Conan the adventurer (animaniacs be awesome too). Modern is next, with my favorites being invader Zim (it cuts the mold with its own unique style though, not sure if it can really be compared to the others)and the grim adventures of bill and mandy. 80s cartoons fall more into the "So bad its funny category", as do many of the hanna barbera age. They are watchable, but none of them really make it into the favorites category.