What is the best covenant for Warlock Shadowlands?
BIS COVENANT FOR AFFLICTION WARLOCK SHADOWLANDS
It's hard to get reliable information on Warlocks atm, but the general consensus for Affliction is that Kyrian is your best choice, as it has strong synergy with their kit, especially after the buffs it has received. That said, the other abilities are all still viable options for Affliction.
Kyrian Necrolords Night Fae Venthyr
BIS COVENANT FOR DEMONOLOGY WARLOCK IN SHADOWLANDS
Ever since Demonbolt began working with Decimating Bolt, Necrolord has been a very strong option for Demonology, especially on Single Target. That said, the damage falls off a bit on AoE, so you're likely better off going with Night Fae for pure AoE damage or Venthyr for solid AoE damage and utility if Mythic+ is what you prefer.