should i leave or save the rachni queen in mass effect 3?

Answer 1

If you saved her in Mass Effect 1, and then again in ME3 she will contribute with some troops to your battle. If you're facing the Reaper one then if you'll save her she will later betray you, and kill some of your troops.

Answer 2

If you saved the Queen from ME1 she will send rachni workers to help you out    and you get EMS points for that. But if you killed the Queen in ME1 and then save Reaper made one she will turn on you and betray you.