i've had three. i'm still unsure which is best- all of them worked well-
the clark zapper seemed the least effective.
the 'auto-zap' that i got works well. here are two websites, one from which i bought mine (i do not want you to think i am advertising- i do not have any interest in any health company- i am fairly new to this)
i also have one from ultimatezapper.com (the ultimate zapper)- it FEELS the strongest- and i've had worms migrate to the surface of my skin underneath the zapper! but it works differently, and so i use both the auto-zap and the ultimate zapper to try to cover everything (i go by the 'little lotta' theory LOL)
the Zapper Forum
has quite a bit about zappers- apparently there are much more sophisticated and expensive models.
there are also $10 models on ebay- which several folks on that forum say work well...