From the originals, I really liked Mike Lu and Og. I loved the setting and idea of kids stranded on an island. The characters were goofy and I enjoyed watching them. I wanted to know more about the island they were on. Plus I liked the theme song. Overall I found the series simply relaxing. It caught my attention and sticked. It wasn't my favorite Cartoon Cartoon, but it was decent.
I also used to watch Time Squad a lot, just to see who they'll meet and what can they do. The animation made the people look funny and reminded me of Horrible Histories.
Apart from these two, I have fond memories of many World Premiere Toons/What A Cartoon Show! pilots that never became a series - almost all of them come to mind, but George and Junior's Christmas Spectacular, Mina and the Count and Kitchen Kasanova were probably my favorite.
From the newer ones it has to be Secret Mountain Fort Awesome and Robotomy. These too I feel failed too quickly. They felt a bit like Adult Swim shows, so that may be the reason, but I wanted to see more from them nonetheless.
Worth mentioning are also Thundercats, but they got what they deserved. Half of the show was amazing, but the other half of it was terrible. I had bigger expectations, so I'm not going to be defending it.