Diablo 1 is a very different game than what comes next. It much more more slow paced and methodical. It has a tenseness to it thanks to the danger and how slow your character moves.

Diablo 2 basically defined the modern isometric action RPG genre. Many fans claim it is still the best entry in the genre and the game is good enough to back that up. It’s got a good balance of speed, action, build customization, and the campaign is fun to play through. You can play it either casually or min max a build. The only major criticism I have is that physical based builds tend to lack compared to magic/elemental builds until you learn your runewords and get decent gear (or twink your character out with gear from another character). Diablo 2 also got a really good looking remake recently.

I can’t speak for Diablo 3 or 4 too much. My experience with Diablo 3 was playing it on couch co-op with friends a few times. It was alright as a mindless beat em up but it didn’t really have any of that classic diablo 2 magic.

Diablo 4 I will probably wait for a deep sale to get. It is an online only game which means that it can be killed or made significantly worse like what Blizzard did with Overwatch. Unlike a single player game, with a game like this you can’t roll back your client version to a version you like if the devs screw the game up. Plus all of the reviews I have seen so far say it doesn’t really have anything over D2. Graphics wise D2 looks about just as good thanks to its recent remake, plus it’s not way too zoomed in like D4 is camera wise.

My overall suggestion - Get D2R on sale for like 13 bucks or something. If you like it then maybe get Diablo 4 later on when its the same price or less.