Good thing we are talking about "Most memorable cartoons from the 21st century" and not anime. So, let's see, shows that marked 2001 and beyond,
1 - Avatar- Last air bender./legend of Korra.
2 - Adventure Time.
3 - Steven Universe. ( dragon ball Z whit emotional moments lol)
4 - Gravity Falls.
5 - Justice League/ JL Umlimited.
6 - Teen Titans.
7 - Over the Garden Wall.
8 - Rick and Morty.
9 - Fairly Odd Parents.
10 - Ben 10 ( OG and all the others)