I am not convinced there is a best fox caller!
There are loads on the market from a few, to hundreds of £££'s and loads of people just use lips, hands greaseproof paper, polystyrene, etc etc, and different land favours different calls..try what you have and see how it works for you
no better answer than that really, because different calls work in different places....try and see, leaving a break in between make it seem like a real animal in distress, is the best advice i can give, i see people too laving digital calls on for 4 or 5 mins solid and wondering why they see nothin