Best Practices for Achieving Realistic Shading in Adobe Illustrator

Without knowing exactly what kind of style you're trying to achieve and in what application, this is a little bit difficult to answer but there are multiple ways you can achieve "realistic" shading. If you want a one-click solution, the 3D effect can get you relatively accurate results quickly, but it's more difficult to stylize in my opinion and can look tacky.

If you're trying to have stylized shading, i.e. shading isn't necessarily true to real life color, gradients allow you to pick very specific colors that fit your palette, plus you can also use the "freeform gradient" that allows for the placement of colors that will blend together on a single shape. This can get muddy very VERY quickly so its best to keep it simple, using as few points as possible(you can scale the influence of individual points) or just stick to linear and radial gradients, creatively placed and shaped to blend into one form. The results of these methods are very much reliant on your own knowledge of form and lighting, so take your time when creating the gradients. Ultimately illustrator isn't intended for "realism" but it can be achieved to an extent with enough patience and practice. 

Protip when using gradients, try to keep the colors you pick analogous to each other or monochromatic, otherwise you risk creating muddy unsatisfactory colors where colors intersect and try to blend.