That's a really hard decision to make!
I really liked
Hana to Akuma. It's not too short and not too long and has a sweet story (and i admit it - it made me cry in the end).
If you like shorter stories, I can recommend
Hanatsukihime. It's setting is very "arabian nightish", it has demons, sweet characters and really beautiful artwort.
Soryanaize Darling also was a good read. It starts like the typical smut manga but if you keep reading you are rewarded with a really interesting and good plot.
Oh, and let's not forget
Gekka no Kimi and
Shinobi Life!
Also
Hana yori Dango was a great read! The artwork in the beginning is horrible but improves a lot in the ongoing volumes. And even though the story has its ups and downs and sometimes drags on just a little bit too long, the story has stayed in my head and I keep rereading the manga on a regular basis.
And a big thanks to everybody who posted in this topic: I found a lot of mangas I didn't know and really want to read