70's and earlier: Nothing holds a candle to the classic Looney Tunes, though some Tom & Jerry stuff comes close
80's: Ducktales and Dangermouse were easily the best, in that they were 2 of my favorites then and are still enjoyable today. Voltron and Transformers didn't hold up as well.
90's: Animaniacs rocks hard. On toast. Batman: TAS is also excellent. Honorable mentions to Beast Wars and The Tick, though Beast Wars took a while to take off.
This decade: Samurai Jack is possibly the greatest action cartoon ever made. Teen Titans is generally very good. Really like Justice League, but I feel like JLU has diluted the goodness on a few episodes with second-stringers who really don't deserve to carry the core team's capes. Heck, most of them aren't even 2nd stringers. They're about 4th or 5th at best. And the more they tap into the DC Universe's version of Greek mythology the more I think the show might drop off my DVR. One or two good lines or moments in a 22 minute episode are not worth this mangling of the source material, IMNSHO.