Not exactly small but the best I've heard:
Fried C5 (long discontinued, were mostly available as kits) used a mass loaded transmission line for mid/bass in a truncated pyramidally shaped cabinet.
Paradigm Active 20 (long discontinued, $1600/pair USD) very similar to Studio 20 v.2, but fully active design with vastly superior performance.
Munro Sonic Egg 150 ($3000/pair USD) composite egg shaped cabinet, external amps (active design).
Sonus Faber Extrema ($70,000/pair USD) uses a passive radiator.