I don't watch TV much, let alone series, but this one I liked a lot and thus recommend: it's called Engrenages and it's a French cops and rob^Wkillers (cum judges and lawyers) series with a lot of bite. (It's got an English title, too, which is "Spiral".)
The fun thing about that series is that all the portrayed people are crooked, from the cops to the baddies to the judges and the land sharks. The good are often bad and nasty, the evil occasionally are humane, and justice is often neither just nor blind.
It's lovely, not a single straight line in sight.