You generally want to take whatever has the highest ilvl, however that starts to matter a bit less once you start getting around 850.
I don't remember the proper names, but there are relics that buff immolate and those are the ones you want. The better quality the relic, the better the buff so it could be worth sacrificing a couple ilvls if it's the difference between 40% chance for immo to crit vs 3% for CB to refund shard(just using those as an example.
If you follow pyromancer in YouTube he recently put out a nearly hour long guide on destro and it's extremely in depth, but there is a section about relics and he does provide specific examples. I'd link it but alas, I'm on mobile :/
Yes, I know that 8% crit and 6% dmg immolate relics are the best, I just wanna know how many ilvls do these relics win over chaosbolt shard refund or whatever else