An interesting question and one that sometimes I do get asked. The answer, as interesting as it may sound is simply it depends.
It depends on need – yes need and not want. The question did say need after all. What are you using it for?
The second point is what is your budget? That obviously matters a lot and there is no point in getting the best camera body if the lenses can not keep up.
Many photographers who do a lot of work actually need or want two camera bodies. I have more than two, but generally only take out two at a time. ( I have medium format, film, full frame digital and crop sensor digital )
My primary work is bird photography – likely 85% of what I do. However I also so events, sports, landscape, architecture and astrophotography.
For astrophotography the D750 or D850 are the top – in fact the D750 is one of the best made by anybody for astrophotography.
For bird photography the D500 will win, the extra reach the crop sensor offers, the reasonably high resolution and the incredible shooting speed and unlimited buffer.. and don’t forget one of the best auto focus systems in the industry. In fact the D500 is likely one of the best, if not the best crop sensor camera made and is without a doubt the top bird photography camera. But it also needs to be coupled with a lens to match!
For closer birds, larger wildlife, landscape and even many sporting events the D850 comes into play as a camera of choice. Although for fast sports the D500 can also be used very well due to the shooting speed!! The D850 can come close in shooting speed but only with the additional cost of the battery grip and D5 battery to complement it.
Some may ask “what about the D5”? Well I don’t own a D5 and frankly while a very good camera I have not chosen it for a few reasons. First it is quite expensive. Second the resolution is really no higher than the D500 and there are times the added resolution of the D850 is great. Yes it has the best high ISO of the Nikon cameras and very likely one of the best made – all be it at a cost of low ISO performance. Yes it has better high speed auto focus than either the D850 or D500 and is one of the best made – matched perhaps only by the Sony A9. Yes it has great shooting speed and incredible buffer. But lets break that down.. it has the high shooting speed and buffer – even better than the D500 but lacks the added reach of the crop sensor. Resolution is no better than the D500 and not as good as the D850. So what you choose is always a compromise!! If I only shot birds and sports then perhaps the D5 would make sense versus the D850, but for landscape and architecture the D850 easily wins.. so it is always a compromise.
Do you need these cameras? Of course not!! You can take incredible images without them. Go out with what you have now, shoot and enjoy.
The image above are a few of the cameras.. but not all. There is also a film camera which I still use in that photograph as well.