The All Seeing Eye. Aka the All, the Source etc
Because it contains everything and everyone. Working with it is like working with all spirits, Gods and Goddesses all at once. And that attracts them individually… may find one approaching without asking.
Queen Lilith.
What a lovely question…
At this point… King Paimon… he calls to me, I think, but I don’t feel ready…
Prince Sitri and Duke Focalor (can’t choose one unfortunately). When I’m finally able to make them get along, that is.
Oh shit…while I am certainly torn by the individual beings I have worked with (if we count landvaettir along with my ancestors, all the gods and the recent demons, the list is pretty long as each have been invaluable) I would have to say the crow spirit I merge with whenever I travel to the homes of other spirits. That way I can still visit and learn from the others, even if I am not technically working with them.
Lol! It’s like monotheism in left hand path! I choose lucifer! Why? Even i love hades and all greek pantheon, but lucifer is perfect balance, light and dark at the same time! For example hades is too dark for some people, he tend to pull you into a darkness! But lucifer NO, he can guide you step by step to know about yourself, he is good for beginner and advance! Lucifer is perfect, if you dont really like a darkness aspect, you can invoke lucifer as archangel, if you prefer darkness aspect, you can invoke lucifer as archdemon
Not sure if I’ve answered this before but I’ve actually started to change my opinion on this for a while.
I would honestly have to take Belial just because of his connection to other legions and other members of the “hell” hierarchy.
King Paimon 
Belial.
But seriously, I’m so glad I don’t have to do that. I enjoy working with multiple entities and not having some jealous god tell me what to do. I just don’t need it.