Please help me in choosing the best camera and why?
The best choice for one person may not be the best choice for everyone else. The word "best" means you will get many different responses. Based on nothing but reading about photography gear, I would recommend something like the Nikon D810, although the file sizes are huge, the sensor is well suited to portraiture. This camera is expensive, so when you ask for "the best", you're going to get something expensive.
If your budget won't allow for that, then I would recommend the Nikon D700. Even a used copy of this is probably better than many other cameras.
What lens? Ordinarily, I would say a longish lens that renders skin very well, but since newborns cannot stand or sit up, you're going to be slightly above your subjects, looking mostly downward onto them. This means a long lens is going to be a problem. So maybe I would choose something more like a 50mm, such as the Nikon AF-S 50mm f/1.8 G, for instance. It is fairly good at rendering human skin, but the focal length should be considered as only marginally acceptable. Other lenses could be better, but we really need to know more about you and your budget.
There you have my concise recommendation; a Nikon D700 (used) and a (also used) Nikon AF-S 50mm f/1.8 G.
Then get one or more speedlights, some modifiers, light stands, etc., and learn how to use all that stuff.