Actually, my favourite thing about the show is the brothers' relationship, but I'm not sure if you'd put that under ship - ship being normally romantic.

ALL OF THE ABOVE.

In the first three seasons it was the SL, Dean, and Sam & Dean's relationship.

But during S4 it turned more towards Cas for me, and Destiel. Though I of course still loved the SL / Dean / Sam&Dean.

Destiel became my OTP so really they're my everything, but I do love Cas and Dean separately with all my heart, and adore Sam too.

CASTIEL

Well, characters. That's what usually gets me really hooked on a show, characters I can fall in love with and want to keep watching their journey. There have been shows where I hate all the mains (not this one, of course) but keep watching for side characters I love.
My fave thing about SPN is even though it only has 2 main characters, 2 main recurring characters, it still has so many unforgettable side characters who have come and gone. Although Crowley is my fave of all (2nd fave TV character ever). But so many side characters as well: Gabriel, Rowena, Bobby, Rufus, Bela, Ruby, Meg, Balthazar, Lucifer, Gadreel, Mary, Charlie, Garth, Claire, Jody, Ketch, etc.
Storyline is extremely important but without interesting characters the story doesn't matter at all imo.
Of course one particular ship IS a a huge reason I love the show so much and the main reason my sister convinced me to watch it (well rewatch the first half of the first season since I saw that when it aired but didn't keep with it)....OTP of all OTPs for me....I don't think I even need to name said ship :D

all, but most Destiel <3

Sam is all I care about, especially now that they don't do monsters anymore and all the heaven drama is so boring.

^