And yes, they are indeed alien invaders.

I really hate endless slashers... It's like if murderers in horror movies had to deal with people from some games they'd eventually just get bored and vow to never kill again before even making their first kill.

I voted
agile- dremora
Quickly decided-beast
strong- Atronach
and
Invincible tanks for order knights.

I only chose invincible for order knights because well... they're made from pure order, and they feel no pain. I can't imagine them moving fast, or being strong because that usually requires some degree of ingenuity, free thought, and adaption in order to be effective. They're also the servants of someone so powerful that he was cursed, so not the weak options either... Maybe a weakness for the knights, like they die if you do a pretty dance?

Dremora seem kind of anger prone, and with a short temper. I could see them hurling themselves at me in a bloody frenzy.

Beasts are beasts, there simply to be put down... and tear you apart if in numbers.

Atronachs appear mostly big and lumbering. They kind of ARE their own attacks, so it'd make sense that they'd be most in touch with their strikes, and want to end the battle quickly, lest they exhaust themselves.

Dremora- vanilla
Beasts- cannon fodder (that's what invaders would use them for)
Atronachs- strong
Knight of Order- cannon fodder (since Order Priests use obelisks to spawn them)

Added: Order Priests- a bit more dangerous than Dremora

Dremora: Strong
Beasts: Fast(minus Xivis, they are strong)
Atronachs: Strong
Knights of Order: TANK!!!!!! Pills here!

I'd say it's best to avoid tank-like enemies as much as possible. They can be used, as long as it's not too often, otherwise it gets tedious. Fights that take forever to finish because of some guy has an insane amount of health is not something I'd want to run into often.

What i would like to see are some real boss fights at the top of all the oblivion towers that are unique; rather than just the usual suspects.

Now, that's a great idea.

Yeah, http://www.tesnexus.com/downloads/file.php?id=22320, and "http://www.tesnexus.com/modules/members/index.php?id=125368" styled boss fights would be great.

(ie: scripted fights, or fights where your enemy might be nigh-invincible, but there are ways to easily kill them without a 20 minute long slash fest.)

Dremora: Voted strong. Although they're just "ordinary" soldiers, they're daedra generally belonging to Mehrunes Dagon, so they should be fittingly difficult.

Beasts: Fast and Agile. Beasts in mortal armies wouldn't be the same (just cannon fodder), but we're again speaking of daedra being used to assault all of Tamriel.
If Xivilai fall under this category, they're the last lines of defense in Mehrunes Dagon's army, and should be the deadliest of them all.

Atronaches: Voted strong, but I'd prefer it to be per atronach. Storm are strong as well as quick and agile, fire are quick and agile spellcasters, and ice are just plain strong with some frost spells on touch.

Order Knights: Invincible tanks, but not in the boring sense. They should be incredibly strong due to spells, and otherwise have plain above average health. A self-cast spell should have something such as shield and reflect damage plus reflect spell, then a ton of offense from either spells or damage. Offense isn't boring and will mean more involvement from dodging attacks via movement, whereas the buff can be dispelled with either an enchantment or spell.