Kizaru didnt know about Drake, so probably not even the high rank know about the SWORD, and as they exist even before Kuzan left and Akainu become Fleet Admiral, they leader have to be some old generation marine.
Garp seems the most obvious, but i dont think he is the leader, he is like Luffy, and dont seems to be the guy who do things undercover. For me, the SWORD dont have a leader, they could be rouge marines that want to reform the Navy/WG from inside. Garp could be a in touch with them, or even be a member, and thats how Coby got to become one of them to.
We could think about Sengoku too because of Corazon thing, but he dont seems to go against the will of the governament as we see in Ohara incident.