The International Marblehead (M) class is probably the most exciting to sail with its moderate size and light weight combined with high manoevrability and stability. Essentially a box rule with minimal constraints. A modern M will usually be made almost entirely of carbon fibre - hull, foils, spars and fittings.
The class was the first to adopt hi tech construction in the early 90s and development since then has been limited. This permits almost all boats built since then to be compete well in modern fleets.
The class is quite often referred to as the "Formula One" of radio sailing.