There is no such thing!! Honestly, if you were to ask 100 different die hard predator hunters this question, you' get about 100 different answers. The best call on the market, is the one that is working to bring in predators. One day it may be one call, a week later it may be a differant call.
This is such an array of differant calls on the market, all of which will work sometimes, and most will work most of the times, if the caller simply puts emotion into his calling.
Also, a caller needs to take into account what it is that the predators normally feed on in his/her area, and usually, that is what you want to mimic. This is not to say that a jack rabbit sounding distress won't call a coyote, or bear, where there are no jack rabbits. BUT it stands to reason that if a predator has had experience with hearing a dying cottontail, then from then on, it will respond to that sound with " a free lunch" on it's mind. How ever, if a predator hears a sound that it has never heard before, it may very well respond because of curisoty.
After all of that, some of my favorites are the Verminator Tweety and Physco Tweety, Tally Ho, Cronk Killer Jr., and a few of my own custom calls that I hae built and decided to keep, most of which all open reed calls!