Just off the top of my head- two books I've read semi-recently that I thought had great messages:
"After the Fall (How Humpty Dumpty Got Back Up Again)" and "The Bad Seed"
The Selfish Giant, especially the Lisbeth Zwerger version. I could never read it to my daughter without tearing up! I'll see if I can think of others.
The Bernstain Bears were also favs of mine, a bit wordier. 
Mercer Mayer's Little Critter series as well, though they have much less text.
All my links: https://APHOTICMOTH.carrd.co/
As a kid or as an adult?
Benji Davies, Grandad’s island. Which I still struggle to read out loud, after loosing my grandad a few years ago. Beautifully written and illustrated.
The shrinking of treehorn which I read as a kid, is about a kid who is shrinking and tries to tell the adults around him but they’re all too busy with their own problems. Brilliant book illustrated by Edward Gorey.
Also recently Andy Stanton and David Lichfield’s, When I was a child. Which is about the magic and wonder of childhood told by grandma remembering how the world use to be when she was a child. It then switches as grandma gets sad about getting old and grey, the child takes grandma on a journey to show her how it is still full of wonder and magic. Again It’s so beautifully illustrated and written, while using Andy Stantons surreal and humorous take on the world.
Also the promise, by Nicola Davies and Laura Carlin, it reads out loud really well. It’s about a girl who lives in a hard mean city. She steals a bag off an old lady, while she’s stealing it the old lady makes her promise to plant what’s inside the bag. The prose and the illustrations go so well together. It’s a story that will stick with you.
This is a really hard question.
I love Shaun Tan's book, especially, "The arrival", "Tales of outer suburbia", "The red tree" and "Rules of Summer"
I also started to collect wordless picture books recently, my favorites are "sidewalk flowers" illustrated by Sydney smith, "the journey" by Aaron Becker, "The farmer and the clown" by Marla Frazee. "Flotsam" by David Wiesner. "Flora and the Flamingo" by Molly Idle
Shaun Tan's books are really adult books in picture book format. My toddlers are not into them yet. But all the wordless books I mentioned were very popular with my 2,5 years old toddler. I remember reading "The farmer and the clown" every night for about a month before she moved on to other books.
There are so many good picture books out there, I keep finding new books I love every time I end up in the library.
@peteolczyk said in Which children's books have most impacted you?:
As a kid or as an adult?
Both! Either! I think good kids books will impact adults just as much, if not more, than when we read them as kids.
Thank you everyone for your suggestions! I'm loving them! Many I've not heard of before, but am now anxious to get my hands on! And many old ones that I've read but had forgotten!
Keep 'em coming! And feel free to suggest more as you think of them!
I love "Miss Rumphius" -- it's such a good read aloud book and beautiful message. I wish I'd written it
Also, "Bloom" by Doreen Cronin. I cry every time I get to where the fairy says to the girl "Tell them there is no such thing as an ordinary girl." Tears, like, every single time.
I second "After the Fall."
@Kasey-Snow same here I love reading everyone’s answers. Great question Kasey
@Kasey-Snow The Adventures of Tin-Tin by Herge. Mr. Men books. The BFG. Asterix was my all time favorite, the Gaul underdog fighting the Roman's with his loyal pal Obelix and clever little dog Dogmatix - who loves trees. That's what I was reading as a kid.
@CLCanadyArts I LOVED serendipity books!
@Kasey-Snow I also loved Chris Van Allsburg, Peter Stier, Steven Kellogg, and Tommie dePaola , as well as the alphabet books by Jerry Pallotta. Also Graeme Base... too many to name I guess.
And one of my all time faves to read to my kids is/was TheTale of the Three Trees by Angela Elwell Hunt. I get choked up every time I read it.
@xin-li my 6 year old boy really likes rules of summer. He spends ages looking at the pictures and the text is just about right for him to read out loud.
I really love Shaun tans work too.
@chrisaakins I love tale of three trees. I remember the year my mother gave that to me for Christmas.
@CLCanadyArts I agree both of these series made an impact on me as well.
@Kasey-Snow i really love The Paper bag Princess
I just checked out 20 books, most of them coming off of this list you guys have helped me start, thank you so much! I'm looking forward to digging into them this weekend.
As always, feel free to post in this thread as you think of impactful children's books or as you discover them!
Just to update this list for anyone else looking for impactful children's books: I just read "Great Big Things" by Kate Hoefler (with gorgeous illustrations by Noah Klocek) and "Seed and Trees" by Brandon Walden, and both were really good! The second definitely had a more intentional takeaway, but I liked the build up in Great Big Things to a very simple but true statement.
Miss Nelson is missing!
But its very old at this point! Lol
@Laurel-Aylesworth bloom arrived at ours today all the way from America. Woohoo
anything by Barbara Reid (Two by two and Foxed Walked ALone might be my favorites but also her Night before christmas one) or robert Munsch/michael Marthchenko (love you forever as one example but I love Martchenko illustrations even tho he didn't do that particular book!)
Guess how much I love you
Click, Clack , Moo was so funny!!!
The Cow who clucked - love those illustrations!
The Broken ornament by Tony Diterlizzi
my kids loved The Bone series of books and we all loved Tintin too. And Calvin and Hobbes (goes without saying really!)))))
I'm sure there's more but I am forgetting , might write more later, these ones just popped to top of my head!
@peteolczyk It's so good!
I love "Miss Rumphius" -- it's such a good read aloud book and beautiful message. I wish I'd written it
Also, "Bloom" by Doreen Cronin. I cry every time I get to where the fairy says to the girl "Tell them there is no such thing as an ordinary girl." Tears, like, every single time.
I second "After the Fall."
