In recent years, I have generally "moved on" from Poirot-Wallander type of series (usually 1 episode - 1 solved case) to more sophisticated and intertwined series where events take place in parallel and with flashbacks, and for full comprehension, one should watch all the episodes in sequence. It take more time and dedication, but there are more versatile characters and more complex situations where one can ponder on and over more intensely.
Line of Duty is a really fine example of the above. The two series - so far - have been really catchy to follow, there is both mind-work and twisted turns, and they are more or less equal (in my opinion, One has a bit stronger performances thanks primarily to Lennie James as DCI Tony Gates, but Two has a bit meatier script). But anyway, I did really enjoy the course of events, the often thin line between cops and crooks (what about its actuality?), and nice round-up of things that really took place. I would juxtapose Line of Duty with e.g. Engrenages (France) and Ørnen: En krimi-odyssé (Denmark).
I am looking forward to next seasons already.
PS. 3, 4 and 5 are meaty as well, please proceed :)
PS. 6 has somewhat less interesting outcome, but interesting twists and turns still visible.