Hard to decide, they all have some pros and cons.
Asari.... well, they're Asari.
Salarians could find a loophole, Drell have that 'perfect memory' thing going (useful to find holes in the other side), Elcor... just for the lulz. Krogan are intimidating enough, and maybe the fact one is defending you would be enough to make the jury give an innocent ruling so they don't get killed... All depends on the situation, nature of crime. Cannot give a good answer without scenario.
And to whoever said Geth, I agree. So much. Simply because it's a freaking robot lawyer. (But Geth are kinda hated, so that could make people mad).