20 year vet experience, TL;DR: 8'th is the best so far.

The editions you're talking about were great, although flawed. The game itself was not hard to learn, but the learning curve was substantially harder to grapple with for those editions. While I do look back fondly on the games I played and the good times with friends, this newest edition of the game is by far the easiest and fastest and most lore-friendly version to date. I think it's down to 3 reasons.

1. All units now use the same statlines. Previously vehicles became a cumbersome mess to explain to newbies (AVs, firing arcs, vehicle type, damage table). Today everything is a unit, and pretty much everything is written with 1-2 special things about it, but it's never much info that you couldn't fit it all on your phone screen. The toughness of a marine isn't the same as a vehicle, but the application is universal now.
2. The game is works faster because of the streamlined rules. Age of Sigmar used to get a lot of heat for it's 4-pages of rules... But it laid the foundation for 40k's 8th edition in making a very digestible game. I would go further to say that 40k is not a simple game; it is a subtle game. Obviously the meta rewards certain combos, but the game is won by the nuances of little things you do more than just 'I brought a knight.'
3. Tyranids and bloated army lists. Some armies required you to invest in 3-4 books to play. Granted, these days you will likely see someone using multiple codexes for some 2K games, but that's a result of something like Marines + Inquisitors, or IG + Knights, or Harliquins + Eldar.