Well, on another topic I mentioned animes, so here I'm adding the best mangas with the best closure in my opinion:
Anime:
Fullmetal Alchemist
Clannad: After Story add the specials too, like Kyou-hen
Working!!!
Boku dake ga Inai Machi
Manga:
Ashita Dorobou (Not very popular, but a solid execution for a sci-fi/fantastic romance seinen)
Death Note (It needs no words)
Fujimura-kun Meitsu (The most awesome ending for a gag manga, as you don't really know if it was axed because how well the author pulled it off)
Full Metal Panic! Σ (Just follow with this the second raid anime and pray for the new anime to actually happen)
Karin (well it is one of the few romantic comedies that actually has a proper ending)
Molester Man (Don't judge it by its art or sinopsis)
Onani Master Kurosawa (idem Molester Man)
Sugar Dark: Umerareta Yami to Shoujo (It was a nice dark story with a few refreshing plot developments)
Zetsuen no Tempest (You can watch the anime instead of the manga, but don't miss the special manga chapters)