I have no idea about the fastest discrete transistors, but the fastest transistors tend to be transistors embedded into an IC design. The moment you have leads, parasitic capacitances from the packaging, etc, you loose most of the speed.
I know for example that working sampling oscilloscope circuits in the lab are pushing towards being able to view 1THz waveforms, but unfortunately they only work with a super-fast risetime edge generated from a Non-Linear Transmission Line on the same chip located microns away from the sampling circuit.
Richard.