What is the strongest being it is possible to summon?

I'm pretty sure Spider Daedra or something such as that. Possibly some of the summons you can buy from the dude in the Mage's guild who is in the Praxographical center.

the strongest I've come across is either a daedric lord, nether lich or a Xiviali

Its not always the strongest, but the most effective..

In Vanilla oblivion no SI, the most effective up to level 15 or so is the clanfear.. fast, tough, strong good time limit 45 secs I think on the spell.

Highest level / magicka cost, liche, gloom wraith and Xiviali in that order.

What about in SI because I have 100 conjuration but the highest level thing I can summon is a lich

My character is level 20 and has used all the summons. Her fave is her clannfear, Spike. He is nasty mean and crazy fast. She has a combo spell that makes her invisible and summons Spike for 30 seconds.

She can then charge her foe, safely leading Spike right into their face while she circles around behind for a bow shot to the back.

It also makes for a wonderful panic button if she is surprised or cornered. From the enemy's perspective, the fragile bowgirl simply turns into a scary clannfear.

She uses the other summons, but Spike is her fave.

I have three main ones Xavier the Xiviali, Donald the Dremora lord and Larry the Nether lich

Donald's my favourite

In the Shivering Isles, upon completing the main quest, you receive [Not sure if this is a spoiler or not, so don't read it if you want to find out for yourself]