NOTE: Within each series, you'll have a choice of frequencies. Which one you pick depends on the specific geographical location where you will be performing, because each location has its own unique set of radio transmissions and such that could interfere with your system. There are some handy websites online that can help you find the right frequency choice, OR you can contact Audio Technica, Sennheiser, or Shure directly and their representatives will likely be able to assist you.
As for the microphone element itself, many theatres automatically buy the Countryman B6 ($260) hair/lapel mic, or E6 earset mic. These both offer great build and sound quality, and many professionals swear by them.
However, there are many alternatives nowadays that offer comparable quality for much less money. We highly recommend the MMAudio wireless series ($115-$150). We tried them out and haven't looked back. The lapel/hair microphone is actually smaller than the Countryman B6, and thus less visible to the audience. We have found the sound quality to be excellent.
Now one question you might ask yourself is, "Should I buy an earset mic, or a lapel/hair mic?" Our recommendation is to go with the lapel/hair mic. They are less visible to the audience, offer greater flexibility in where you put them on the actor, and they are much more robust. The big problem with the earsets is that they have a bad habit of snapping when you adjust the boom to the actor's face. The hair/lapel mics have no stiff parts, and thus nothing to break.
Overall, we recommmend spending as little as possible on wireless equipment, particularly on the microphone elements, which are exposed to makeup, sweat, etc. You will likely have to replace the mics fairly frequently, especially if you have youth performers wearing them, and/or anyone other than trained professionals putting them on the actors.
We attach the transmitters to the actors by means of an invention of our own design.
We buy 1.5" elastic like THIS, and cut it into strips long enough to go around each actor's mid-section and double-knot in front.
We buy heavy duty webbing like THIS, and cut two strips a couple inches longer than the width of the transmitter. We put one strip on each side of the elastic belt, pull the elastic tight, and then tape all the way around the webbing, leaving room in the middle for the transmitter. We then fold the ends of the webbing over the taped part, and tape around it again.
That's it. That leaves you with a stury, comfortable mic belt. Slide the transmitter inside, in between the webbing and the elastic, with the belt clip catching onto the webbing. Tie the elastic around the actor's upper mid-section with a double knot, and you're good to go!
Alternatively, you can buy a neoprene belt like THIS ONE.