basically i always feel empty everytime a manga ended (with some exemption on the axed one, i feel anger)
but if you wanted something that "i feel sad it's end".
https://mangadex.org/title/414/sket-dance have best ending, it's end but Shinohara-sensei sure make grand closing to it's serialization.
https://mangadex.org/title/6282/cuticle-detective-inaba maybe not the best in term of storyline, but the end is good, i still have few disappointment but it's good.(there's no english chapter on MD, try another site)
https://mangadex.org/title/21161/ore-wo-suki-nano-wa-omae-dake-ka-yo this love story full of twist surely left me empty after it's end.