  [ABRIDGED] The knowledge of the regular component of the Galactic magnetic
field gives important information about the structure and dynamics of the Milky
Way, as well as constitutes a basic tool to determine cosmic rays trajectories.
It can also provide clear windows where primordial magnetic fields could be
detected. We want to obtain the regular (large scale) pattern of the magnetic
field distribution of the Milky Way that better fits the polarized synchrotron
emission as seen by the 5-year WMAP data at 22 GHz. We have done a systematic
study of a number of Galactic magnetic field models: axisymmetric, bisymmetric,
logarithmic spiral arms, concentric circular rings with reversals and
bi-toroidal. We have explored the parameter space defining each of these models
using a grid-based approach. In total, more than one million models are
computed. The model selection is done using a Bayesian approach. For each
model, the posterior distributions are obtained and marginalised over the
unwanted parameters to obtain the marginal 1-D probability distribution
functions. In general, axisymmetric models provide a better description of the
halo component, although attending to their goodness-of-fit, the rest of the
models cannot be rejected. In the case of disk component, the analysis is not
very sensitive for obtaining the disk large scale structure, because of the
effective available area (less than 8% of the whole map and less than 40% of
the disk). Nevertheless, within a given family of models, the best-fit
parameters are compatible with those found in the literature. The family of
models that better describes the polarized synchrotron halo emission is the
axisymmetric one, with magnetic spiral arms with a pitch angle of ~24 degrees,
and a strong vertical field of 1 microG at z ~ 1 kpc. When a radial variation
is fitted, models require fast variations.
