Will finish collecting every single exotic weapon very shortly when variks calls me back to pick up my lord of wolves. Otherwise, the answer to your question: Red death - hard mode crota and occasional crucible. Truth - first exotic so I have strong ties to it and try and use it for all void engagements. Thunderlord - I need a few hundred more heavy weapon kills so I try to use it whenever I do something not that challenging, or when arc burn. Ice breaker - ya know. Thorn, last word - all the time in crucible. Vex - occasional crucible.