I personally love the games; they provide a satisfying gameplay loop of murdering hoardes of enemies to get better equipment so you can kill even stronger enemies. Using new builds and discovering new attack combinations/synergies is a lot of fun as well!
Diablo 1 has a great atmosphere but is pretty lacking by modern game standards. If you decide to play it, I'd recommend adding the Hellfire expansion so you get an extra class and the ability to sprint.
Diablo 2 is brilliant and lets you party up with up to 7 people instead of the max of 4 in D3 and D4. It's definitely a tougher game compared to the newer releases.
Diablo 3 is a fantastic game now that they've added the Adventure mode and rifts which make it way easier to tackle quests and grind for gear. It's also almost limitless on its difficulty unless you invest hundreds of hours into a character.
I've been jamming on Diablo 4 since it came out, and it could work out to being the best in the series; however, it's clear that it has a bit of growing to do. The skills are fun and fair though, and the five classes are diverse enough to give you plenty to chew on as you discover your preferred play style.
Obviously I love them, but I could see how others might find them repetitive. I'd snag D3 on sale (or it's probably discounted now) and decide if you feel like D4 would be worth $70.