My absolute go to system is the Lectrosonics 411 Digital Hybrid receiver paired with a UM400 transmitter - add a Countryman B6 lav and some quality cables and you've got - in my opinion - the best sounding, most reliable wireless. Just today, I used two sets in some pretty fierce conditions (heavy RF, lots of metal obstruction and stray signals) and not one single dropout in 8 plus hours of shooting. Plan on paying about $4000 per channel for everything.
I couldn't agree more. The Lectrosonics 411 actually sounds like it's hardwired!!! I had heard that from someone else and did my own side-by-side comparison. I could not tell the difference! Also, the advantage to going with a digital-hybrid is that you keep the digital audio but send it using an analog carrier which, if you find yourself dealing with a bad RF situation, will at least give you some sort of signial (as opposed to an all digital system where the signial is ether there or it isn't).