Dave and Ava's slogan is "Rediscover Nursery Rhymes!" Compared to many of the other related series on YouTube, Dave and Ava seems to be on the higher-quality side. The animation is excellent and the singing and music is generally top-notch. They have very long videos, too. While they repeat the same ones over and over again, it means you don't have to change the video for awhile. This seems to be the case with most of the nursery rhyme videos, but Dave and Ava's are the ones that capture my daughter's attention the longest.
If you have a child ages 1-6, or younger, and you're looking for a good video series for your child to watch, I would definitely recommend this series. They also have a mobile app (currently only available on the Apple Store), which is $20. It's a bit steep in my opinion, but for ad-free entertainment for potentially months or years, it's not a bad deal for Apple device users.
Dave and Ava currently put out new videos every Thursday on YouTube.
If you know any other really good YouTube video series, let us know in the comments!