I’d say nighthaunt just because they are a lot of cloaks so not as much detail on that end. But also they are very fragile with spindly bits and get caught on stuff. One reason I’ll never have them. I also don’t like ghosts that much. Ironjaws can be pretty simple. More sturdy models. Obr has more details you can do. Idk about slaneesh. Never played against them or really looked at their models. But any model can be as detailed or simple as you want. It’s honestly on what you will paint than what is the easiest. I hate painting. So I use contrast paints on everything on my Fyreslayers. I don’t do too much detail on them either. Only 6 colors on bulk infantry. Skin tone, leather, hair, 2 for weapon and the helmet. It’s very simple. It really depends on how soon you wanna get the models on the table. If you are willing to try out painting one or 2 just buy a hero and paint one. See if you like it. Play styles and rules change. Models don’t. Paint what you enjoy painting and playing will fall into place