Which of these Sigma lens should I get as a macro for my nikon dslr?

That really depends on what you plan to shoot. The 105mm is better if you plan to take pictures of flying insects and can't get really close. The 50mm is nice but you already have one. The 70mm is also really nice because its the middle ground and can function as both a nice macro lens for insects and flowers and such as well as a nice sharp portrait lens. 70mm is ideal for me at least for portraits because it gives a flatter appearance. Another thing you want to look into is the magnification. 1:1 is nice but I'm not sure which ones if not all can do 1:1 macro because I'm a canon fan. ess your use and see which works best for you. Also keep in mind about the aperture, wider is usually better for lower light and depth of field control. Personally, me, I would go with the 70mm because its a good combination between a portrait lens, and for macro shots.