Personally, I use Core Hounds for solo stuff.
The ferocity leech is far better than the spirit beast heal and core hounds are more tanky due to their damage reduction skill. The leech applies to your pet as well so it’s even better as your own personal tank and has crazy self healing in AOE situations. Core hounds also deal more damage when they are being melee attacked because they have a reflect type ability. Spirit Beast hardly offers you anything for soloing in comparison to Core Hounds, just a heal that you’ll hardly ever bother using.
Anything for normal/heroic/mythic dungeons, doesn’t really matter. I go for ferocity Chimaeras for the leech (which means the healer doesn’t have to help you as much) and the extra AOE damage ability.
I always use ferocity in Mythic+/Raids (unless it’s been predetermined by the leader that I should use something else). Specifically Chimaeras because they have an extra AOE damage ability. Spirit Beast can be useful in Mythic+ but the heal is pretty minor in the grand scheme of things. It’s rare that it’ll significantly help the group. As with other instanced dungeons, better to have the leech so the healer doesnt have to help you as much and can focus on others.
Devilsaurs for PvP because they reduce the healing the target receives by 25% automatically and that debuff has 100% uptime. It is essential for PvP and every hunter should be using it. Riverbeasts/Direhorns/Hydras/Lizards can be used instead if you want a Tenacity pet for PvP for the extra damage reduction cooldown, they also have the healing received debuff. I prefer having the Leech from Ferocity though. Scorpids/Carrion Birds/Ravagers/Wasps can be used as a ferocity one as well, they also have the healing received debuff. It doesn’t have to be Devilsaur.
Summary of what I think every BM Hunter should have in their toolkit:
Core Hound - Soloing
Chimaera - Dungeons/Raids Default
Spirit Beast - Dungeons/Raids Situational
1 of Devilsaur/Scorpid/Carrion Bird/Ravager/Wasp - PvP
1 of Riverbeast/Direhorn/Hydra/Lizard - PvP