I love Cedric Diggory.
His character was marginalized by JKR, but I don't fault her for this, the characters that she created are all wonderful and unique, but she couldn't possibly flesh out everyone. Fortunately we have the fandom to do it for us.
Here are some of the Cedric-centric stories I've come across.
Touching the Sky by
florahart - lovely Cedric/Charlie story. She starts us at the beginning, in Ottery St. Catchpole with a very young Cedric and wonderful, carefree Charlie.
It was within by Twinsuns - This story represents Cedric so well. It is a gen-fic that just gives lovely insights into who Cedric was as a boy on the verge of manhood.
Not Yet by Dazzledfirestar - Cedric/Oliver - Just a little snippet of a story that has you wondering what happened between the two of them that makes Oliver yearn to join Cedric twenty years later.
I've posted four Cedric-centric stories and drabbles myself. Having had to follow canon and kill off Cedric in two of my stories, I am determined to write an AU where he lives.
Beyond the Words (Cedric, Luna, Harry/Draco - Drabble)
Last Dance (Charlie/Cedric, 8,400 words)
Dragonlover (Charlie/Cedric - Drabble)
Finally, my Harry/Draco
story book, Twist of Fate, actually started with this conversation between Harry and Cedric. I never intended it to be a hp/dm story when I wrote it: in September, 2011 I had just started reading slash Harry stories and it got me wondering what it would be like for Harry Potter to go through the realization that he is gay, and hoping he would have had someone to help him/support him through this journey. That is where the story of Twist begins:
( Conversation between Harry & Cedric (excerpt from Twist of Fate) )