The number one reason I read Noblesse: Cadis Etrama Di Raizel.
Never, in all my years of reading all genres of fiction novels, short stories, epic poems, mangas, etc, or in all the movies or shows I've watched, have I ever seen such a self-sacrificing, loyal, and absolutely astounding show of character as I've seen in Noblesse. Here you've got the most powerful being on the whole planet from the get-go, who only uses his power at the last minute to protect those who care for him, and also to punish those who abuse their power even when it pains him to do so.
I can't write enough praise about Rai so... Frankenstein, his servant, is the second reason I read this. His devotion to Rai is awesome, and he's super powerful and wonderfully sadistic... really the opposite of his master.
The other characters are great too, even the "regular" human kids. I love the story and the plot. There's the traditional nobles on the 'good' side and the up-and-coming Union on the 'bad' side, though neither are really good nor bad.
It's not a mainstream shounen, but it is a shounen. Again, the MC is the most powerful guy on the planet, so he easily destroys those in his way. But he doesn't want to, he doesn't scream and yell about it, and it hurts him.
There are some flashbacks, but what makes them so great is they're just long enough for the reader to understand more about the present but yet not get bored with and stuck in the past. Some even only last a few panels, but much is told with just those few.
And the last bit of why this is my absolute favorite manga/manwha/etc is the humor. It's original, all of the characters show a bit of it, and it's fairly clean. It also comes at unexpected moments. Humor is a must for me and Noblesse does a fantastic job of delivering it.
Also... this is the ONLY manga/manwha I have completely read more than once. ^^
edit: Shounen is only as good as its length. I really loved this series years ago but it needs to end! Rai is still awesome and the art is beautiful but this webtoon is past its prime. Even the most loyal fans get worn down from repetitive fighting.