The Borg, from Star Trek
We are one, not single, far different from the rest
The Borg are a collective entity. Usually, a single Borg drone has no true sense of self.
We can regenerate from battles in a sort of nest
Borg ships are large cubes, which contain repair stations that look like alcoves in a hive. The mechanical parts of Borg can easily be fixed and enhanced after being damaged.
We can adapt, assimilate, use it for the best;
The Borg mission is to assimilate all organic life into their collective.
We live in this galaxy, but in a quadrant, far far away
The Borg originate from somewhere in the Delta quadrant.
So can you guess this place I stay? And what I am that I say?
You are from the Delta quadrant, and are either in the Borg mothership or in a traveling Borg cube. You are Locutus and you speak for the Borg.
If you're so smart, who was taken away from us and reunited back at their original home?
If it's not Locutus (Picard), Data, or Hugh, then you are probably referring to Seven of Nine. She was actively rescued from the Borg by the Federation, and was returned back to her original home in the Alpha Quadrant.