GaAs is the most suitable for a linear and efficient RF power amplifier. But in the same time is the most expensive process.
Most of the WCDMA PA’s on the market still use a standard linear amplification, and sometimes using adaptive bias or a DC/DC converter on Vcc to improuve the efficiency at low power.
It seems that only SiliconLabs release a commercial PA with CMOS, besides, it's a nonlinear one.....
Since the square characteristics if I-V, the linearity of cmos PA is quite bad so that it's not suitable for WCDMA......
For a stand-alone PA, maybe the GaAs is the most mature one.
The advantage of SiGe BiCMOS is that it can be integrated with other blocks(transceiver/baseband...)