HH I believe. Take a look at Ulaby et al. , IEEE Trans. on Geoscience vol. 16, 1978, p 294 & vol. 17, 1979, p 35.

Dear Mateusz A. Kedzior. Your simple question got a valuable answer on HH polarization, but I feel that needed a bit more because the question and answer does not touch the essence why HH would be so valuable for observing soil moisture (SM) changes. In my understanding the reason for choosing HH is not focused on the soil moisture. The reason for HH is because of the linear type of polarization components. Linear polarizations are HH and VV. Linear polarization variables are usually much larger than cross polarized HV and VH and the more HH and VV determine the polarization index (PI). The larger PI gives more relevance to the dielectric losses in soil, what relates to the water content. One may even ask for the reason of existing polarization components. My answer is that polarization appears due to electromagnetic losses in media as a result of the softest (elastic) impact of the propagation medium on a photon of electromagnetic wave. Other polarization components still play very important roles in sensing which are caused by the move of target, instrument and in media. Possibly you will not need that for your work. However, it would be good to learn tutorial materials on polarimetry for PolSARpro available from http://earth.eo.esa.int/polsarpro/tutorial.html, including few examples from Part VI "Do it yourself". 

Yes, usually HH polarization is the most used and the best for soil moisture retrieval.

Anyhow, it depends on many factors. For instance, the ASCAT soil moisture product is obtained by C-band and VV polarization, with good performamce in many places of the world. The next scatterometer (SCA) will have also cross-polarized (HV) acquisitions, and improvements are expected.

To sum up, it's still work in progress.

Dear Luca

I believe that HH  polarisation  is most valuable for observing soil moisture change. Take a look at :

Ulaby et al. , IEEE Trans. on Geoscience vol. 16, 1978, p 294 & vol. 17, 1979, p 35.

- Hadria R., Duchemin Benoît, Baup F., Le Toan T., Bouvet A., Dedieu G., Le Page M. Combined use of optical and radar satellite data for the detection of tillage and irrigation operations : case study in Central Morocco. Agricultural Water Management, 2009, 96 (7), p. 1120-1127. ISSN 0378-3774

--  Hadria R., Duchemin B., Jarlan L., Bouvet A., Dedieu G., Baup F., Khabba S., Olioso A. (2010). Potentiality of optical and radar satellite data at high spatio-temporal resolutions for the monitoring of irrigated wheat crops in Morocco. International Journal of Applied Earth Observation and Geoinformation, 12S, S32–S37: (DOI : 10.1016/j.jag.2009.09.003).

if you want, i can send you the last two papers by email.