The fact that it has only 6 episodes per season makes it great - no wasted scenes.
Alfie's exchange with Tommy in season 3 about his son might have been the best acted 5 minutes that i've seen on TV ever.
Yeah i found all episodes except maybe 3-4 to be crucial to the plot, nothing wasted or boring. Always something happening.
I like how it didn't transition in time too much except the war flashbacks. It was directed like a novel.
Tom Hardy did an insane acting job here, he was by far the best talent on the show.
Lots of twists to keep things fresh.