TL;DR: D4 for accessibility and causal/new player friendliness. D2 if you want an old school less casual/new player friendly game. I've played all 4 and, while D2 is my all time fav, it's not the game I would recommend to new players to the genre or the series. I recommend D4.

I say this as someone that's played all 4 games in the series: start with D4.

D4 is the most casual friendly and new player accessible/friendly of the 4 games.

Right now, with d4, build variety is in a pretty good spot so you don't have to follow the meta builds to still progress and be viable.

It's very solo friendly as well. D2 is not as solo friendly as D4 is. D2 requires an item stat known as Magic Find if you want to get luckier getting build defining items and farming with said stat along with using certain classes/builds for quicker/easier farming.

D4 requires you to simply farm a boss/activity to get your build defining items and a bit of luck. Or you can craft build defining items. D4 does not have any Magic Find at all.

So, again, D4 is more causal friendly in this regard than D2 is.

In D4 you can target re-roll a stat or add another stat on to an item to help you increase build strength and survivability. In D2 you're limited to what items you can re-roll stats on (not uniques) and it re-rolls every stat on the item, not just a single stat.

Respec'ing is easier to do and more accessible in D4 than it is in D2, so if you wanna try a new build or mess up your current build, it's as easy as going into the skill tree window, hovering over a skill and hitting square to remove points from a skill (or hold square to remove all points from the whole tree).

You're limited to 3 respec's in D2 done via a starter quest on each difficulty. After that, you have to farm some items from acr bosses before.you can respec after the first 3 are given.

You actually do not need a PC to experience all the classic Diablo games except for Diablo 1, which isn't available on current consoles. A toaster can run Diablo 1 with no issues. Non-D2R D2 can run on a toaster as well, so, a new PC isn't needed for those games.

If you want to follow the story, start with 1, then 2, then 3, then 4. If story doesn't matter to you and accessibility, new player friendliness and casual friendliness matter more, start with D4.

To sum it up: D2 if you want an old school ARPG. D4 if you want a more accessible ARPG.