Gaians. University and Peacekeepers are also not bad choices. It's also especially important to remember that the Unity was launched because humans were wiping out life on Earth (and, if the Usurpers complete the Ascent, your forces return to Earth to find a ruined and dead world in the epilogue).
Also, you can't choose any of the new factions; they formed after Planetfall (well, except the Progenitors, who obviously weren't on the Unity).
Aki: Former Peacekeeper Talent, killed in battle with Spartans, merged with a University sentience experiment
Roze: Former leader of Morganite intelligence-gathering
Domai: Former Drone in the Human Hive, rebelled
Cha Dawn: Foundling discovered by Gaian empaths
Svensgaard: Rogue Spartan ship captain
Before you ask, it's all in the written summary of the SMAX plotline.