A high pitched rabbit or some sort of rodent/mouse.A simple bulb squeaker is hands down the best call for reds.
I only hunt for reds when I have a partner.Place the shooter 50 to 100 yards downwind of the caller.By doing it this way when the fox circles the caller the fox will give the shooter an easy shot.
You can also use the terriean to your advantage.If you know a foxes habits you can cut him off before he makes his circle downwind.As shown in these pictures
These pictures are spots me and my parnter took reds from.Same setup and everything
Black dot-Caller
Red dot-shooter
Red line-Wind direction
Blue line-Foxes approach
Tips
Dont call a real lot
Dont put the main light beam into its eyes only keep the halo above the foxes eyes
They usually go to the highest point in the field before making there circle downwind
If you see one the it disappears dont get wooried he will be next to yuor shooter in a minute or 2
They often hangup when they do give them a couple quiet lipsqueaks