Rafael Nadal is best known for wearing Nike shoes, and in fact he endorses this brand and is sponsored by them. He has been seen wearing the Air Max Court Ballistec, for example - you can see this and other products associated with him here (you have to scroll down a bit to find the shoes).
In fact his name is associated with a whole range of clothing, rackets and other tennis accessories including several other shoe types, all sold by Nike - you can see the range here. However, obviously he can't personally use or endorse all these products. You can easily find images online of him wearing a specific brand, but of course many famous sports people wear certain brands because they are paid to, rather than because the brand is their personal favourite. So it will be difficult to find out what kind of sports wear Nadal really thinks is best!