The assortment of wrenches available for purchase is enough to make you dizzy. But if you're only going to buy one, a big adjustable wrench is the one to get. It's able to handle nuts and bolts up to an inch wide.
Adjustable wrenches come in long or short shafts.
The short shafts will allow you to get into tight places, and the long shafts will give you more turning power. One disadvantage with adjustable wrenches is that sometimes in the less expensive models, the jaws will slip. And if they're loose, what will happen is once you get your wrench around the head of the nut or the bolt, it will slip and come over the back side, and it will mar the outside of the bolt. So, when you're purchasing an adjustable wrench, always make sure you get one with smooth screw action. Also check the jaw, and make sure that the jaw is real tight.