Star Fleet Enterprises

Originally a branch of Lone Star Combine, Star Fleet Enterprises was their pirate hunting division, but it's now a separate company. Star Fleet Enterprises still hunts pirates as their most primary occupation. 
Star Fleet also patrols sovereign space, for nations that cannot manage their tasks alone, and they escort transports. 
Star Fleet is perhaps the biggest provider of any security service when it comes to off-planet services, and still have an incredibly close relationship with Lone Star Combine. Star Fleet buys personnel from Lone Star, to sit as Security and Law-enfocement on space stations and large space crafts, after retraining them. 
They also have a working relationship with Echelon Multistellar, based on the fact that it was too expensive for Echelon to train their own space marines but Star Fleet lacked the proper candidates for the training, producing sub-standard space marines prior to the establishing of the arrangement. 
Now, Star Fleet buys contractors from Echelon and retrains them as Space Marines and Echelon then outsources their space craft, and space marine needs to Star Fleet. 
For the same reason, you never see Star Fleet and Echelon on opposite sides of a contract. 
Star Fleet Enterprises is the biggest player in the field without any rivals to speak of.