Which camera: Nikon D610, D800, D800E?
Which one is best in your opinion? And why?
Take your blinkers off, look at the competition, and work it out for yourself.
The one that costs the most is usually the best.
But since you're asking this question, I don't think any of them will be best for YOU!
These are very advanced cameras.
If I were you, I'd look at a Nikon D3100 or Canon Rebel T3.
You have to walk before you can run.
The yellow question mark hit the nail on the head. When I re-entered the DSLR area, having retired with 2 1/4 negative cameras from my professional wedding work. I looked at the best brands, Canon, Nikon & Pentax. I chose the Pentax and for the most important reason that it was/is the most comfortable camera for "ME" to hold, to view through, and to see myself taking pictures with. I did not care that it was the "entry level" Pentax DSLR at the time. Also I might add that for a "grab it quick SLR", I had used Canon cameras all my life.
In the end (over 3 years ago) I'm still pleased with my decision. These are the points that stood out to me in addition to the viewing and location of controls, was the fact tht stabilization is in the body of the camera. Enabling me to purchase used lense even dating back to the "film" days. You can't do that with Canon or Nikon. In additon the Canons viewfinder was just too busy especially with it's 5 focusing squares. I liked the one focusing square which can be moved around, especially handy when your are doing "framed" shoots using a tree or something else to frame your photograph.
Remember the most important part of your your decision is YOU and how you shoot!
Best for what? I think you are going to be way over your head with either of those three.
D610
My next camera is going to be a Nikon D800E which will be replacing my old Nikon D3
Which is best?
They are all excellent and since you have to ask, you may not be ready for one quite yet
You have to walk before you can run with the big boys.
Buy a nice Nikon D3300 and learn how to produce brilliant images each time you press the shutter release. Don't worry about buying DX lenses for now, they will work on any Nikon full frame dSLR when you are ready to step up.