After recently getting the D4R-II receiver for my quadcopter I'm ready to call it my favorite I've used or seen.
Why do you ask? You don't? I'll tell you anyway.
It's "4 channels" but has CPPM (collective PPM) support out of the box for all 8 channels. In other words you'll only need Vcc, Gnd, and CPPM to connect it and get all 8 channels. Software is way simpler with CPPM and you have one of the smallest RX out there.
It's a real telemetry model. Out of the box the FrSky TX will beep when it's going out of range. You can hook up off the shelf 'sensor hub' modules and use expensive LCDs to see telemetry like battery voltage and position. Bleh. My setup uses this telemetry option as a transport layer for arbitrary data sent from MultiWii or ArduPilot directly. It's not high throughput but it is highly useful. More on that in a bit.
It's hackable to use TTL serial logic. FrSky uses RS-232 inverted logic for all telemetry communication. That sucks. They used mosfet transistors as inverters (see this diagram from sparkfun's tutorial). I fixed this inverted logic problem by soldering nice wires to the pins where TTL logic comes off the STM32F103 µC it uses. Now I don't need a level converter on my quadcopter!Read more.