Normally you have for every nut size a dedicated wrench size. However, if you don’t want to store 10 differently sized tools and don’t need a wrench on a daily basis, an adjustable wrench is an optimal solution. Most adjustable wrenches are not fun to use: they easily slip away and you don't have a great lever.
Knipex changed that with the Pliers wrench. It is easy adjustable with one hand, yet holds the object tight. The best adjustable wrench out there and the only one that does not feel as a compromise in use. Made in Germany.