So I know I'm very late to the party here but I just finished the last season of Dexter last night and wanted to give my thoughts.
Season 1 - Started off a bit strange but it quickly caught my interest after that. I called the doctor being the ice truck killer but had no idea he would be Dexter's brother. I remember thinking "man how are they going to top this for 7 more seasons" at the time and boy was I right. They blew their load way too early with using the biggest villain first.
Season 2 - Another fantastic season. The divers finding Dexter's kills was a great storyline. Really wish these were the final seasons instead of the first two though. Deb and Lundy was a bit weird at first but it grew on me. Dobes was the best side character on the show for me. He and Dexter had serious acting chemistry. I enjoyed the Lila stuff as well with Dexter exploring his dark passenger. She was super hot so that was a plus.
Season 3 - This one started off very interesting with Dexter having a potential friend he can confide in but quickly went downhill for me. Miguel wasn't an intimidating villain and the war torturer guy had very little background and character other than that he was a war torturer guy. Dexter saving Miguel's brother at the end was a nice touch though.
Season 4 - A big improvement from season 3. I enjoyed all the Trinity stuff. He was a good villain that you almost feel bad for in the end. The family balance stuff was interesting to see with Dexter murdering on the side at first but was getting old by about season 4. Rita's death shocked me at first but I'm glad it happened now. I don't think I'm alone in wanting less family problems.
Season 5 - I think a lot of people disagree here but I enjoyed this season. Jordan Chase was a good villain and I enjoyed seeing Dexter put himself back together a bit. The main plot was still good but what I had issue with is this is about where all the side stories start becoming useless. Angel's girlfriend is out of the picture at the start and Angel and Laguerta just all of a sudden have a thing when there's been nothing even hinting at this before? Then the next season they just abandon it.
Season 6 - The last good season for me. I loved the religious theme. Brother Sam was a good character and so was the villain. Didn't see it being only him doing all of the sacrifices coming at all. This season had a clear theme and direction and they stuck to it. The bad seasons (3,7, and 8 for me) do a terrible job of this.
Season 7 - This one seemed to be written by different writers and then just mashed together. The kashka brotherhood was good at first, I enjoyed the gay mob boss guy. But it ultimately led to having zero impact on the story other than a bit of character development for Dexter. In season 6 or 7, Laguerta is all of a sudden a villain type character as well which was strange. I enjoyed Deb finding out about Dexter but I hated the whole her loving Dexter storyline. Completely unnecessary and just kind of gross honestly.
Season 8 - Mostly a chore for me to get through. I liked the Vogel stuff at first but it quickly got old for me. I was thinking she was the one pulling the strings which would've made for a good final villain. I liked the final kill scene with Oliver but everything after was pretty bad. Deb dying hit me hard. I think Dexter riding into the hurricane with a long monologue would've been a much better ending. There was zero point to showing him alive as a lumberjack but I didn't hate the ending as most seem to.
Overall, a very good show that went downhill after season 4 or so but I still found seasons 5 and 6 to be very good. I don't hate the ending as much as most do but I'm definitely not going to pretend like I loved it. I liked Deb's character a lot but the last few seasons got a bit old with how much they centered the story on those two's relationship.
I think the show really suffers from the side plots/characters no longer having meaning/being interesting after season 3 or 4. I think this show would've really benefited from waiting to use Dobes death, Dexter's brother, and the bay harbor butcher storyline until the last few seasons. These were the best parts for me and to use them all in the first two seasons was too quick.
Seasons ranked: 1, 2, 4, 6, 5, 3, 7, 8 with 1 and 2 being more or less interchangeable