I'm curious in your opinion what is some of the strongest standing stone combinations would be? I just started a new character to go through dawnguard again with and plan on using it in my late game. Opinions?

Lord and Atronach I think. Can't remember what I was using.

Also for power leveling, lover and Mage/warrior/thief.

imo atronach and lord

the one that lets you raise an undead army

Which is the Aetherium Crown

Umm..... I can't seem to get the crown to work. I wear it, and I choose warrior stone, then lover stone but it doesn't show up as warrior being under active effects. How do I make the Aetherium Crown work?

Well I have had the Steed Stone on for most of my time in Skyrim (I love that extra 100) so i'm not giving that up.
I might get the ritual stone just for the undead army side.