Top 5 Video apps

Sesame Street by Sesame Street (Free!, iOS, Google Play, Amazon)

All the World: Better Learning by George Media (free to download and sample, subscription for full access, iOS & Google Play)

Avo! by Playdeo (free to download and try, pay to unlock full app, iOS only)

Epic! by Epic Creations, Inc. (free for trial period or with classroom access, otherwise subscription, iOS, Google Play, Windows)

The Elements in Action by Touchpress (paid app, iOS only)

Honorable Mentions:

BrainPop by BrainPOP (free to download and try, subscription, iOS, Google, Amazon, Windows)

Wizard School by Duck Duck Moose (free, iOS only, no longer being updated but still has a lot of great content)