What is your favorite age-appropriate book?

My son is obsesed with books! He's only 18-months but advancing fast, so it got me thinking about what I should be getting him next... and when. What are some classics or new releases (it's been a while since I was a kid) that you swear by? I'm open to anything from age two to sixteen! What are some great books to read together as he gets older? What books should I encourage him to read himself, and at what age?

Thanks for your recommendations!