Okay, so let's clarify a couple of things:
First there is not really any trap properly called a "leg trap"... what is generally called by that name are properly called "coil spring" traps, or sometimes "foot hold" traps, although people confuse the term "foot hold" with "dog proof" traps.
Secondly, a properly sized coil spring trap catches the critter across the paw pad, and should not do any damage to the foot... it will make the toes numb, etc. though. There are even specialty coils springs called offset and padded coil springs, that have a softer catch to be even more safe. So it's generally not the use of the trap that is cruel or unethical, it's the misuse of the wrong size trap that causes issues.
I've caught and released many critters in coil spring traps, including some incidentals such as domestic dogs, and even a bald eagle once, and let them go with out harm to their foot or leg. Using a properly sized trap for the target animal is key.
But to answer the OP's question, the most effective (best ) trap for catching a fox is a coil spring trap... but the use of these traps typically takes a bit of experience and skill to use effectively, and there are regulations that might apply to their use depending on the location.
Cage style traps can be used for fox, but it can be much more difficult to get them to commit.
https://articles.extension.org/pages/10468/tools-and-techniques
https://thefurbearers.com/trapping-and-wildlife/types-of-traps
A more introduced primer. There are still regulations on foothold traps in certain states. In mine they are regulated in their spread. With our conservation office they are considered cruel especially if not checked more often than the regular 24 hour period. One ended my career as a game warden but that was a very large bear trap.
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