Helen Mirren has been a favourite since 'The Long Good Friday', and I find I love her more with each year. I thought she was incredible in part 1 - ageing, drunken, emotionally screwed. What a character arc. Mirren gets more interesting and attractive as she gets older IMHO.
The plot was probably less compelling than the ground-breaking Prime Suspect 1 - I'll never forget that series. I watched it on holiday, on a tiny set in a cottage in Cornwall, I think an hour a day over four days (or something like that). I was appalled and gripped. For most of the story you thought she had got the wrong guy, and so did her underlings. Without the context that has built up subsequently there was no preconception that it would all work out for the best.
In this series the focus is more on her own inner demons in her last few weeks before retirement, and the plot revisited the father/daughter relationship in multiple guises, with both daughters and fathers being bereaved and left without offspring. My guess is that Tennison's daughter-substitute Penny will end up as the murderer, but this isn't a spoiler, just a wild guess.