Without a doubt, SPD. The premiere was one of the best PR has ever had, and it had A LOT of good things going for it: a good cast, interesting characters, a big universe, high production values, and a compelling story. But after the fourth episode, nothing happens. They just billshit for THIRTY FREAKIN' EPISODES. And then they end up screwing up the little plot they had (Aisynia, anyone?).
SPD could've been the season that just blew everyone away, but it just ended up being one of the worst, most poorly written seasons of PR.