Mine is Thierry Henry, although that's mainly because I am Arsenal 95% of the time, so get a lot of practice taking free-kicks with Henry. Van Persie is usually very good at them when he is in the team. Often, I let him take them instead of Henry when they are given in dangerous positions around the box. But i've scored the majority of free-kicks with Henry.
I have also found Cristian Chivu excellent at them, but that is using my improved stats for him
Generally, I find left-footers better at free-kicks. Recoba, Roberto Carlos, Mihailjovic, van Persie, van der Vaart, Chivu etc all take mean free-kicks,