Series 1: Great telly. Excellent cat-and-mouse dynamic of one serial killer baiting another, interspersed with Dexter trying to balance his cover life with his other side, with some great villains of the week (Carl from Ghost convincing people to top themselves for example!)
Series 2: A great expansion. The main baddie was, admittedly, perhaps too hateable in the wrong way, but this was basically series 1 but with more. Frank Lundy was such a likeable character and despite him technically being the antagonist, you found yourself rooting for him in a way.
Series 3: Not as good, but enjoyable. Helped to show that Dexter really isn't an antihero no matter what he tries to say.
Series 4: I've said it before, I'll say it again. Fantastic slab of telly. Lithgow is a disturbing and scary villain, not just when he's smashing heads in with a hammer or convincing victims to jump off of ledges, but also when he's got his nice face on. Lundy returns, Dexter has a lot of character growth (admittedly this is where his "I'm an emotionless husk" thing REALLY starts to make no sense) and everything is well-paced.
Massive coverage, Golden Globes and other awards, big prspects for cast and crew all at this point. However, this is also where the final two of the original three members of the showrunning/creative team leave the show. And...
Series 5: SEX CRIMINALS. Because fuck subtlety! These are POWERFUL BUSINESSMEN who HATE WOMEN and LOVE doing sex crimes in order to FEEL POWERFUL! Such one-dimensional villains who had all the character and articulation of a Wolfenstein 3D enemy. I'm surprised they weren't nazis too, actually. There's a lot of blatant mid-season panicking in the writing too, like bundling off Dexter's stepkids and moving him into his apartment again as if nothing had happened in the last four series. The female lead is so unlikeable too. "What do you mean you won't murder these people? I'm purer than pure, lighter than light - it's even in my name! You have to do what I say!" Yeah, fuck off love. The only shining light is Peter Weller, who plays a wonderfully shady corrupt former cop.
Series 6: Yet more panicking and retconning, most blatant of which is the unnerving-in-the-wrong-way "It doesn't matter that she wants to have it off with her brother, it's fine, it's her stepbrother!" which was quickly backpedalled. The baddies were interesting, and the dynamic of good vs evil-but-flipped had legs, but ultimately all I remember from this are a few visual setpieces and Colin Hanks being really good. I want to see him play a T1000 based on this performance.
Series 7: The clusterfuck truly sets in! Decisions and attempted retcons result in some outrageously out-of-character performances and it really feels like the show is coming apart at the seams to say the least. Also Yvonne Strahowski was so rage and-or somehow apathy inducing that I was delighted when she died in my first Mass Effect 2 playthrough.
Series 8: Dint watch it. Sounds even more nonsensical though.
It's amazing how quickly and severely the show went, as Tommy Cockles once put it, "from talk of the town to whisper of the village". You hardly ever hear it talked about now, and Michael C Hall's career, which seemed to be on a vertical trajectory, just kind of... didn't skyrocket like it should have. I reckon if Dexter ended after series 4 he'd have been massive. Like, MCU massive.