For effective, high velocity , accurate rounds for foxes, you won't beat a .222. Not as big and powerfull as others in the .22 centrefire range, but about the flatest trajectory you can get, using a 55 grain pointed soft point round, is heavier than the .17 so will be less affected by wind etc, and ammo is cheaper than .22 250 etc.
Obviously, if your after deer as well, a larger calibre will do both jobs for you ( as has been stated earlier) but an out an out fox rifle .222 in my humble opinion. If you have say £1000 to spend, use £400 on a good rifle and spend the rest on a very good scop