The original DCAU (Batman: The Animated Series through Justice League Unlimited) is phenomenal. BTAS is still unrivaled for US animation, so fucking good. The Batman is average at its best, with the possible exception of the Batman vs. Dracula movie that is about a thousand times better than it has any right to be given its pedigree and premise.
Seconding things mentioned in this thread:
The Real Ghostbusters: It's a bit rough by virtue of being a product of the state of the art at the time, but easily the best cartoon to come out of the 80's. It starts to lose altitude in the later seasons, culminating in a complete collapse when they went to 15 minute episodes to pair up with 15 minute Slimer slapstick shorts, but there's tons of good stuff to be had.
Invader Zim: Not everyone's cup of tea, but I love it. Fucking dark, though.
Bojack Horseman: Stick through the first few episodes until it finds its swing. It's hilarious, has too many sight gags to possibly catch them all, but damn does it get bleak from time to time.
Rick and Morty: Generally good, but extremely scattershot. The latter half of the first season and first half of the second season are the goldilocks zone. Rick's constant vomit drool hanging off his lip is incredibly off-putting.