I think they’re all worth watching, though they are highly variable.
Personal rating is: Daredevil series 2 > Jessica Jones > Daredevil series 1 > Defenders > Luke Cage > Iron Fist.
Pretty much everyone is well cast. Finn Jones is the only disappointment (and really, he’s not all that bad, it’s just the Iron Fist didn’t come together properly), all of the other principal characters inhabit their roles excellently. The supporting cast is frequently fantastic, especially The Kingpin in Daredevil, Jeri Hogarth and Trish Walker (the MCU equivalents of Jeryn Hogarth and Patsy Walker in the comics) in Jessica Jones, Cottonmouth and Mariah in Luke Cage, and Colleen Wing and Grant and Ward Mitchum in Iron Fist.
The shows do a great job of being distinct, but still having some links back to the greater MCU. Some of it is really minor, like Hell’s Kitchen takes an economic downturn due to the devastation of the alien attach in The Avengers, or kids are seen playing in Cap and Hulk costumes in Jessica Jones. But there’s nice links to Agents of SHIELD, where a biker gang shows up there and in Daredevil, and the same special bullets used to try and assasinate Director Mace in one episode show up in Luke Cage as well.
That distinct nature of the shows appears in different ways - They’re all shot and lit differently, and have different core themes. Daredevil is quite violent and dark and nihilistic; while Jessica Jones is about PTSD and abusive relationships; Luke Cage is about the tribulations that can befall communities; and Iron Fist… has kung fu. (and some stuff about dynasty and legacy). The Defenders has a nice visual trick early on where each character’s scenes is shot in their own show’s specific style and colour palette before they meet up.
Every last one of the shows has pacing issues. Defenders does best of all because of the shorter episode count, but there’s still a bit of drag. Daredevil series 2 is in my top slot because it has two distinct story arcs and balances them well.
There’s some really ropey stuff in there. A lot of people single out Karen Page’s voiceover at the end of Daredevil series 2, and it’s pretty bad - but there’s similar moments that threaten to turn you off in each series. Go in with an open mind to the cringe.
Overall, when the shows are good, they’re rivalling Agent Carter Series 1 and the best of Agents of SHIELD - and they never sink to the depths of AoS’s worst moments. They’re perfectly enjoyable genre TV, but they’re no Twin Peaks or Sense8 or The Expanse.