I think Sims 2 was the best of the Sims. All of the charm of that game can't be beaten, imo. But between 3 & 4, I'll have to say 3.
I didn't mind not having open world in 4, because my Sims always stayed at one place anyway in ts3, and loading was slow. Was disappointed that you couldn't make your own hood like 2 though.
The Sims look AMAZING, but they all act the same, and complex emotions was a good idea but ended up seeming pretty faked. Sims had emotions in 3, and even cried over things and got angry in 2, so they can't say emotions are entirely a new thing if they've been there all along. I also love gallery, dislike the too-happy nature of the game, love CAS and building, but dislike the few features and staleness. This game could've been amazing.
In 4, It doesn't feel as "mature" as the others, like it was catered to younger kids and preteens instead of teens and adults. It feels like you watch over their lives like a kind (or evil) overseer, in worlds you can't create. In 3 (and 2), it feels like you're actually playing God.