I'm gonna be the oddball here and recommend D2R. Although I haven't yet played 4 ($70 wtf no thx), I've played the rest, and while I certainly have the nostalgia goggles on a bit, I'd say it's one of the best examples of the genre. The remaster did a great job of adding some desperately needed QOL features that have come about in the genre since its release, and updating the graphics, without detracting from the atmosphere of the game.

As for 3, personally, I feel like it removed too much of the thought, i.e. skills and stats, and put the focus solely on luck, i.e. gear and loot, while also removing some deeper loot customization options; as well as the graphics and general atmosphere being more cartoon-ish and much less bleak, as much better fits the series, imo. But as someone else has said, 3 can be tried for free, and the general gameplay loop is pretty ubiquitous throughout the series: Kill monsters, level up, get better gear, rinse and repeat. So you can really get a feel for if that kinda thing is for you or not.

I've definitely heard that 4 is much more of a return to the "series' roots" if you will, so that might be worth checking out as well.

The OG is great, but it definitely hasn't aged as well, and it's very clunky and a bit slow, but is definitely worth playing at some point. And there are cool mods too add new classes and such!