assuming my conjuration is master, what's the best creature to summon?

gloom wraith
lich
storm atronach
dremora lord
xivilai

thoughts?

Not sure if you are aware of this or not:

http://www.uesp.net/wiki/Oblivion:Spells#Conjuration

Might answer this question and a thousand more.

yeah, I guess I just don't really understand what does the most damage.

it's not just about strength too, you have to weigh the magika cost AND how long the summon lasts.

As far as I'm concerned the longer a creature stays with you, the better. So for my cowardly master conjurer/illusionist/thief a combo spell of invisibility (45 seconds) and summon Storm Atronach (40 seconds) is the go-to spell. By the time the Atronach is gone my magic is fully recharged and I can do it all over again.

assuming my conjuration is master, what's the best creature to summon?

gloom wraith
lich
storm atronach
dremora lord
xivilai

thoughts?

I found the Lich capable of killing just about anything in the game. Whatever it can't (which was rare) I could then kill it with a hit or two.