Decided on a whim to get Sims 4... (been wanting to for ages... shaddup I like them games)
kinda don't like the new layout (I think I last played sims 2?)
From what I've gathered, Sims 4 improved the most in regards to socializing and interaction between other sims, but as far as customization and making your ideal living scenario, 3 beats all.
Yah, essentially they cleaned up many of the errors that made Sims 3 such an inevitable memory-dump. Sims 3 featured an open-ended story mode that tracked each and every Sim in a game-world and what they were doing and where they were at any given point, which allowed for fantastic levels of interactions and progression, but which ultimately created unwieldy amounts of data complications as game-saves progressed (to the point where the game would no longer save or would crash on its own after much gameplay.)
Sims 4 opted instead for Sims 2 type 'neighborhood sub-instances' (instead of a seamless global neighborhood) which would then populate NPC sims for which the player could interact, which improves game performance, but at the cost of that 'Story Mode' progression (and adding the necessity of loading screens when moving across instances.)
This irked the heck out of the Sims 3 faithful, but of course made the Sims 2 faithful rather happy.
The other gripe, of course, many had was that as far as a "base game" was concerned, Sims 4 was very bare. It didn't have many of the features native to even Sims 2 or 3 or even 1, as a base game (no swimming or swimsuits even, and occupations are back to being rabbit-hole type places your Sims disappear off to, as if it were Sims 1 again.)
While it's inevitable that EA will add such functionality in later expansions, many saw this (cynically, but quite rightfully so, I think) as an EA expansion/DLC-money grab--i.e. offer bare minimums then make people buy all the parts later. :/
So it was really a lackluster launch for something many really were hoping would be something more. But insofar as the game exists, the inter-sim interactions are really neat for sure. Whether that creates enough play-ability to keep people interested until the necessary expansions come out is something else, especially with so many other games competing with it. :/
TLDR: People didn't like instances and too bare a base game, but it fixed core issues with the Sims 3 game architecture. If you are willing to wait for expansions, it will probably be very cool (eventually.)