1 is definitely great. At the time of release, it was Truly Great.
2 - Everything RDD said. It was ROUGH for the first 3-6 months as far as "end game" and the expansion pack gave that game added an entire fooking decade of life to it. it also rad(doggyDawg)ically changed the gameplay by introducing synergies between skills.
3 - the former lead designer of the Legion WoW expansion, Jay White, was lead design on 3. The game was VERY dumbed down in terms of skill selection and bonking. The "artificial difficulty" came with Torment difficulty (which was the top tier at launch). The loot tables were fucked and you couldn't get gear good enough to be able to survive. Corpse walking became a regular method of progression.
There's a ton more that I think most people forget was added from the expansion.
https://www.ign.com/wikis/diablo-3/Reaper_of_Souls
4 - Right now it feels like a mix of 1 and 2. Gameplay wise, definitely 2. There are a TON of intricacies that go into skills and how that shit works, kinda like 2. The atmosphere and campaign are reminiscent of 1. End game is sorely lacking right now, but will improve with both Seasons, and an expansion down the line.
The expansion was tremendous for this game as well, from many standpoints. Feel like they fell flat with the Crusader (Paladin-lite, the Diet Coke of Holy - just a lil faith, not quite revenant enough!). The addition of Act V and core design changes were very needed and welcomed. The changed loot tables though, made the game WAY too easy eventually.