Quick look at Sanwa M17, RX-491, RX-492 and thoughts on earlier M12, M12S and the budget MX6 (Sanwa offer a cheaper budget MT44 radio too) plus description of uses, quality and accessories, and some compatible receivers such as RX-472 and Banggood clones.
Sanwa are clearly the top racers choice, fastest response, SSL system, great signal, range, reliability and build quality.
For full SSL functions MT5 or above models
For budget racing 10 model memory MX6 is great target price £80 with 2 waterproof receivers.
Recommended over any other brand, although I appreciate budget can be a deciding factor