TS 4 is my favourite. Since someone asked for specifics, here's why:
1. My main reason is that it's by far the easiest to play stories with. If I want a challenge I play 2, but most of the time, I just want to use the game to play out all of these stories. The fact that their needs aren't that hard to take care about and multitasking is what makes that possible. I use my own imagination a lot when I play and can come up with what my sims should struggle with on my own. I usually don't make them have their dreamcareers right when they age up to YAs, I make them fall in love with people that don't like them back and so on. I even have my own system for when to age sims and banking and stuff.
Would I like those things to officially be part of the game? ...maybe? I like how the things I come up with myself works. Also, not everyone is that hardcore. When I start a new game, I spend a long time creating all the families, making houses and coming up with ideas for storylines. That's not for everyone. And no, I could not have done this with the other games, because taking care of the sims took too much time.'
2. I like the art style. If they would try to make it realistic, I thin kthey would end up in uncanny valley. This way, it looks good and it doesn't look like something that is trying to look real, but can't.
3. I like how neighbouthoods are handled. There are plenty of sims moving around to make it feel alive, but it's not lagging massively. I would have liked it if we had open neighbourhoods, but I think this is okay. For me, it's preferable to only having the lot to move around or having an open world, but no sims anywhere.
4. The toddlers. We got them late, but I think they're the best addition yes. I'm a family player, they are important to me.
And so on. The rest are smaller things that I just genrelly like.
Also, it bothers me how people act like liking 4 isn't valid. Everyone plays the game differently. 4 is definitley more for story players than those who want a challenge, and it's okay to play any of the others if they suit your playstyle better. Just don't tell others that they can't like 4.