ORIGINAL: jester_s1
I don't see any reason to doubt that Spektrum/JR is the best selling radio, mostly due to getting into the market first and the fact that nearly everyone makes bind and fly models that work with Spektrum/JR transmitters. I'm not real clear on exactly what the arrangement is between the two, but the impression I got was that JR started the Spektrum brand when it released its 2.4ghz technology. As for which is the best radio, pretty much all the other of the big 4 have a good reputation with Spektrum/JR not so great. That said, as long as you use a 6v battery pack and don't get one of the odd radios that can't stay bound you'll do fine with Spektrum too.
To be perfectly accurate, only Horizon makes models that bind and fly with Spektrum transmitters. The bind and fly models sold by Nitroplanes, Hobby King, Great Planes and Hobby People (Airtronics equipment) are not Spektrum compatible. There is an Anylink module that translates Futaba and Spektrum and Hitec into the 2.4 signal that Great Planes/Flyzone uses, but not compatible without it.
Also, JR did not start Spektrum. Spektrum was a startup associated with Horizon that did 2.4 on its own and then they licensed their shiny new 2.4 tech to JR.