Depends on your budget, and how portable you want it to be. I would never bring a 24-70 f/2.8 with me unless it's for an actual working contract. Too big/heavy/obtrusive.
It also depends what kind of photos you like to take. If you take portraits with a wide-open aperture, then you'll want to get a prime. I you're getting a D750, then you should already have an idea of what kind of photos you'll be taking.