Definitely Jimmy Neutron. It's where Sheen started out. I didn't expect much from Planet Sheen when I first heard about it. The descriptions made it sound like it wouldn't be all that good. But I actually really like this show a lot. Sheen's character is still the same, and Doppy is very similar to Carl, so their interactions are quite similar to the interactions between Carl and Sheen that were great on JN.
I'm liking Planet Sheen more and more as time goes on and more episodes are made. But one thing I think would make it much better is if every few episodes or so, there would be a scene of Jimmy and the others back on Earth trying to find Sheen and bring him home. Even if they didn't end up finding him, I think an episode here or there with them working hard to get him back would be great.