THE standard in high end wireless is still the Shure U series, but they are pricey. If you want the best, that has been it for some time. This is changing, though, as more and more companies start using the Audio-Technica 5000 series. You will not be unhappy with either, though many guys I know and respect are saying the mics on the 5000 sound a bit better.
I, for certain, would NEVER spec anything less on a rider, if I had to use wireless mics. (They are such a pain in the ass.)
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"Cowards can never be moral."
M.K. Gandhi