In a way, I would have to say the first episode of Season 6. And that's simply because it was the beginning of Chase & Co. just treading water for as long as they could - in order to make more money - at the expense of keeping the story line moving along.

I've said it before and I'll keep saying it until Hell freezes over. The end of Season 5 set up everything perfectly for a regular 13 episode Season 6. The 2 part final season, with additional episodes, wasn't needed and allowed for too much downtime.