My personal list:
Anime > Source:
-Haruhi Suzumiya - LN - Anime somehow remains nearly 100% faithful to the LNs yet takes what was otherwise kind of a silly and meaningless story and elevates it to masterpiece level. Maybe a hot take: KyoAni already adapted all the best parts of the LNs, it's almost like I can feel Tanigawa's creative exhaustion seeping through his words in the later novels
-Mushoku Tensei - LN - The LN just took the web novel and published it. Yep. This was definitely an amateur web novel, a step above fan fiction. But the anime is terrific.
-Jujutsu Kaisen - Manga - The manga itself is a rough gem. The excitement and genius is here. All of Akutami's humor and personality is fresh here and was adapted faithfully into the anime, though the sheer coolness and style is unique to the anime. Also, some of the character designs look cuter in the anime, notably Nobara.
Source > Anime:
-Oshi no Ko - Manga - I personally disliked the anime while liking the manga a lot more. To me, the real substance of this story comes from its characters and their well-written psychologies, which is something that is just inherently more difficult to show in anime format as opposed to writing.
Source ??? Anime:
Now that I think about it, these bullet points don't even belong in this discussion topic, sorry I got carried away nerding out over anime lol
-Sousou no Frieren - Manga - Actually the source itself is terrific. The anime takes that already terrific source and jazzes it up a bit more. The anime hasn't even gotten to the best parts yet.
-Kobayashi-san chi no Dragon Maid - Manga - Bwahaha. They are both extremely good. Though similar to Haruhi, I think all of the best parts have been adapted so far.
Edit: edited for better organization