I'm a little late to the party on BB but finished season 4 last night and had to talk about it somewhere - what an incredible end to the season.
Gus' backstory with Don Eladio, the Cartel poisoning, Walt's speech to Walt Jr about his father, the net beginning to close in on and Walt and Jesse, absolutely amazing. The scene that will stick in my mind forever, however, is the ending of Crawl Space with Walt's crazy laughter in the background when Marie calls up about the hit threat on Hank. That final shot as the camera pans up to the ceiling with Walt's face framed by the trapdoor (intentionally coffin-esque I'm guessing though please no spoilers!) and the music in the background was just an unbelievable finish to the episode.
I can't imagine at this point going back to start again from season one knowing where everyone ends up, it feels like I have way too much invested emotionally in the characters! The way the show portrays everyone getting sucked into the tragedy unfolding is brilliant.