1. Minibombo apps by TIWI s.r.l. (Google Play & iOS, best for toddlers & preschoolers)
2. Pettson's Inventions by Filimundus (Google Play, Amazon & iOS, best for school age (or preschoolers with an adult companion)
3. Thinkrolls by Avokiddo (Google Play, Amazon & iOS, school age and up, I didn't review ALL of the Thinkrolls series, but they are all equally good.)
4. Chuchel by Amanita Designs (Google Play, Amazon & iOS, school age and up, heads-up Chuchel can be a little irreverent -- one of the puzzles involves him peeing on things!)
5. Stop Motion Studio by Cateater, LLC (Google Play, Amazon & iOS, self-motivated preschoolers & school age and up)
Honorable Mention:
Anything by Sago Mini or Toca Boca (these are only "honorable mention" because I wanted this list to focus on smaller app companies and these guys are the gold standard in kids' apps.)
Fiete Match by Ahoiii (Amazon & iOS, but NOT Google Play)
Very Hungry Caterpillar Creative Play by StoryToys (Google Play & iOS, but NOT Amazon)