  Very precise measurements of exoplanet transit lightcurves both from ground
and space based observatories make it now possible to fit the limb-darkening
coefficients in the transit-fitting procedure rather than fix them to
theoretical values. This strategy has been shown to give better results, as
fixing the coefficients to theoretical values can give rise to important
systematic errors which directly impact the physical properties of the system
derived from such lightcurves such as the planetary radius. However, studies of
the effect of limb darkening assumptions on the retrieved parameters have
mostly focused on the widely used quadratic limb-darkening law, leaving out
other proposed laws that are either simpler or better descriptions of model
intensity profiles. In this work, we show that laws such as the logarithmic,
square-root and three-parameter law do a better job than the quadratic and
linear laws when deriving parameters from transit lightcurves, both in terms of
bias and precision, for a wide range of situations. We therefore recommend to
study which law to use on a case-by-case basis. We provide code to guide the
decision of when to use each of these laws and select the optimal one in a
mean-square error sense, which we note has a dependence on both stellar and
transit parameters. Finally, we demonstrate that the so-called exponential law
is non-physical as it typically produces negative intensities close to the limb
and should therefore not be used.
