Depends completely on the software you are using. Blender, Daz Studio etc? If Daz, it totally depends on your preference but Genesis 3 and Genesis 8 are the standard. Most clothing, animations, morphs, characters and poses are made for those two models. After that it just depends on your personal taste and what you are going for.
You will never have a complete collection because your style, story and images will always be evolving and growing. This is true of any art, unless you pull a Thomas Kinkaid and just paint the same image 10,000x.
Assets I couldn't live without: Genesis 8, DimensionTheory HDRI, Zeddicuss Poses & expressions, Fugazi props and scenes, Windfield hair, Out Of Touch hair. Anything by Valery3D is also excellent. Clothing by OneSix is really nice.