Basically the best legendary/ rare farming spot is in the first mission on the moon. Put it in the hardest difficulty, grab a hand cannon or high damage primary and get to the front door with 3 large runic locks blocking it. After you check on the dead guardian, you need to step back a bit and let all the level 20+ thralls charge you. Once they do and you've killed everything except for the ultra knight, you need to die to the knight. You'll respawn right where the gates open and will probably get about 1-2 legendaries every 10ish minutes. Important notes: I recommend using the painted big chief mk 45 hand cannon from the crucible. Do it with a full fire team because it just means a better chance for everyone with no downside. DO NOT KILL THE KNIGHT AT THE GATE. His shots are easy to dodge just do not kill him as he is your best insurance policy.