As an Action game D3 is better imho but… not sure how fair of a statement that is just due to the time gap and the advancement in many areas to allow for that.
As a RPG game Diablo 2 is MUCH better than Diablo 3 because my character feels more unique. As someone put it you’re playing an individual character more than a class in D2 compared to D3
Overall…
Since I still play both I’d have to give the nod to D2 because that is some insane staying power. Don’t get me wrong I like D3 in doses and have put A LOT of hours into it. The combat is good and I really like three of the classes (WD, Necro, Crusader) but Diablo 2 still be playing today, even if a couple times a month, is saying something. I’m also not much of a nostalgia player, for instance, I haven’t jumped to WoW Classic, WC3:R, or the old console systems. I just find D2 to still be an enjoyable game.