Hands down, the best musical artist for kids. I can still happily sing along with dozens of his songs and recently included "All I Really Need" on a spotify playlist for a friend.
The Singable Songs Collection looks like a good one on Amazon just because there are tons of songs included, but I don't know that any collection is any better than any other. I think you're likely to be able to find a lot of content at your public library or on Spotify.
The songs on his "best of" album are maybe the most recognizable: