The first three series of Sopranos absolutely lives up to its reputation. It's the classic period of the show. When I think about how great it was, those are the series I'm thinking of.
There was a noticable drop in quality in series 4 - it felt like the writers had this massive show and an amazing cast of characters but didn't really know what to do or where to go with them. Still good.
Series 5 was a step up again, in part because of Steve Buscemi. It was just more Sopranos doing what Sopranos does so well.
iirc, series 6 was split in two by the writers strike. The first half was yet more Sopranos, which if you were still watching after all that time gave more of the same. The second half of the series was a bit of a mess, with stuff just happening each episode while in the background the overarching storyline led towards that ending. And what a brilliantly memorable ending it was - people can still argue over what happened.
Wire was great for 4 series. I think of series 1 and 3 as the classic story; series 2 and 4 focused on different things and were both amazing. The central plot of series 5 was a mis-step and the focus on the press felt forced in a way the dockers and education system hadn't, but it was a fun ride watching all those great characters doing what they do.
Breaking Bad was finding its feet in the first series, but got better and better up to the explosive ending of series 4. Series 5 was a bit 'what happened next' leading to a tidy conclusion.
All three shows were fantastic in their own way.