I love animation. Though I'm not fond of a lot of American cartoons that have come out these days. The three big exceptions are Adventure Time, Legend of Kora and Gravity Falls. Adventure Time is like 3 year old crack, the cartoon plays with a vast post apocalyptic world and the 10 minute episodes cover a vast array of topics. It's a perfect jumping off point for imaginations and is a rich enough world that it's no wonder a lot of adults get excited about it. I am not fond of the ultra simple spaghetti limbed people, but I respect it as a way to put out quality episodes quickly and more importantly be a style imitable by small children! In it's 7 season, I find it kind of hit or miss, but I'd recommend anyone give a curated list of episodes a go.
Legend of Kora has gorgeous animation and while I found the first few episodes of season one annoying in terms of immature characters, the plot has gone exciting places and the third season looks to be even better than the last two. It's been one of my go to holiday gifts for friends.
But my favorite of the three is by far Gravity Falls. It's animated by the same studio as Adventure Time, but the character designs were done by someone totally different and thus the art is a whole lot more pleasing. The plot is consecutive, the humor witty, a lot of the themes spooky but not scary and the characters grow in interesting ways. While it's easy to see why a 12 year old would love it, I think it's equally as easy to enjoy at 33.