Zu def 4 are they the best speaker made?

When someone says something is the best or among the best of anything, that statement only has meaning if there is context.  What else did the person hear and find wanting compared to that product?  There is no way for anyone to have seriously auditioned anything but a small fraction of what is available, but still, it would help to know what else was heard and why those do not measure up.  

I have heard the Zu Def 4 and they are, in many respects, very good speakers--very clear and dynamic sounding.  I like that kind of sound myself.  But, if someone thought that the transient attack was too hard and brittle sounding, I would understand that sentiment too. Not that many commercial speakers are as dynamic as the Zu, so I can understand if someone is swept away by hearing them.  But, it would be interesting to know how many other systems with that kind of dynamics and clarity the OP had heard, e.g., Classic Audio Reproduction field-coil speakers, a good vintage Western Electric horn system, modern horn system with ALE or Cogent or Goto drivers, Acapella speakers, Horning speakers?  This list can go on and on.

Evaluating components at a show can be a bit tricky.  If the sound is really good, you can be sure that the speakers are at least capable of delivering good sound.  But, if they sound bad, it is hard to know if the set up is the cause.  If the same speakers sound bad at different shows and in multiple rooms, that does give you a hint.  I have heard the Def 4s at several shows and in many rooms.  Often, it does sound quite aggressive and unpleasant.  But, in a few setups, it sounded quite good, which, to me, means it is capable of sounding good.  

For some reason, most of the setups I heard used powerful solid state amps.  I would think, given the efficiency of the speaker, it would be something that could be matched to lower-powered tube gear (my preference).

Johnk nominates the 80+ year old Shearer system as the best ever made.  It certainly is a contender, as is a number of Western Electric systems, and more modern clones of such classic systems.  I have been listening a lot to custom systems being built these days by Deja Vu Audio utilizing an assortment of drivers from Western Electric, YL, UPC, etc. I like what can be done with such drivers.  The current build that is being fine tuned utilizes 18" Jensen field-coil woofers (reconed with original, unused cones and pleated surround), a WE 713a compression driver and WE 32 horn, and RP302 tweeter, and a vintage WE crossover.  I wish everyone could get a chance to hear this sort of system.  It would certainly surprise those who hate horn systems based on their exposure to Klipsch or common Altec systems.