Sigma or nikkor lens.which is better?

I'm confused between sigma and nikkor 24-70mm f2 and sigma 24-70 hsm f2.8

also suggest me some must have lenses

Sigma, it depends on your field work, fashion, wildlife, landscape, commercial, they all have different lens for the light. It's the light that is important

The Nikkor is also a f/2.8 not an f/2.0 and I'll give the nod to the Nikon on a full frame camera (D3/D700 class). Keep in mind that the Nikon is 3-4 times the money of the Sigma, a much bigger price spread than is the 21st Century norm.

If you don't have the money to afford a Nikkor 24-70mm f/2.8 then the Sigma doesn't do a bad job for an after market lens.
The Nikkor is designed specifically for Nikon cameras and with the crystal nano coating works extremely well. I own and use this lens on a daily basis and can confirm that it is an excellent choice for any aspiring photographer.