I've always considered myself bi-bro, but as I've been writing these posts, I realize how much more I lean towards Sam nowadays. And Season 5 is where that started. We knew in S4 that Dean was as responsible as Sam for starting the apocalypse, and we started to see then how they'd been manipulated into it, but S5 showed us just how deep that manipulation went. And yet Show has rarely passed up an opportunity to place all of the blame at Sam's feet.
Other than that (and a few clunkers, but every season has at least one or two of those), though, I loved this season because it was all about the bond between the brothers and how that bond was strong enough to save the world. It seems like in every episode, one of them was being hurt (or even killed) to manipulate the other, and yet they held firm--eventually. Sam's faith in Dean in "Point of No Return" took my breath away, coming as it did after Dean's lack of faith in him. That little wink Dean gave him when he decided to tell Zachariah to stuff it was magnificent, and when Dean told Sam he was a grown, well, overgrown man and could make up his own mind about trying to take on Lucifer, that might have been the most maturity Dean's shown in the whole series.
And I know I should be wishing that "Swan Song" had ended the way Kripke intended, with both boys in the Pit and the show at its peak. But I can't help but be glad that it's gone on for so many years after that, because it's brought me to so many of you. :)