Is there a best covenant for an assassin rogue. Does it matter what one I join once I’m in shadowlands?
It matters if you want best DPS in raid/M+/rated PVP yes.
According to this, best one is maldraxxus if PVP.
for raiding/M+ its a bit complicated:
For PVE and PVP I’m pretty sure most rogues go Mal at the higher end.
Mal is really good in AoE but I think Night Fae for single target for the ambush/shadowstrike procs. I’m not 100% sure on that so correct me if I’m wrong