I´d say FWL, Fedsuns or the Magistracy. Why? Because one can ally with the three at the same time
I agree.
Allying with the Federated Suns, the Free world League and the Magicstracy of Canopus at the same time (allowed by their corresponding feelings among each others) will provide you with 2 special, high-tier, end-game Faction Flashpoints out of the 5 available (i.e. the Davion and Marik ones), access to the corresponding factions' Quartermaster's Shops (surely the Davion & Marik ones, the Centrella's too - if they have them), as well as full contracts' and shops' access in as much as 28 allied star systems, to which you may add the 24 Arano-controlled and the 36 independent systems, while bestowing upon yourself the enmity of the Taurans, Liaos, Kuritas and Steiners - only the first two of whom do actually control any star system in the Aurigan Reach (even though, admittedly, they manage to control near almost half of it, between the both of them).