Personally, 
I'm gonna have to go with (In no particular order):

- Deadman Wonderland
- Soul Eater
- Medaka Box

Each of them told there stories in an enjoyable entertaining pace, knew what they wanted to be, and didn't really switch anything up. The characters (For the most part) each went though a fair amount of growth and showed noticeable changes by the end. Also, the ending of each, in my opinion, gave a fair amount of closure. 

Honorable mentions include:
- Shaman King
- Mahou Sensei Negima!
- Katekyo Hitman Reborn
- Kuroko no Basket
- Histories Mightiest Disciple Kenichi
- Beelzebub
- Naruto

Underrated Manga in my opinion that are complete:
- Cage of Eden
- Psycho Busters