This feels very cheap to say, but I'll say it anyway. Steam tanks. Particularly the "Alter Kamerad". More barrels = More shots = More lethality?
Yes.
They are the most magnificent example of Humanity's expertise and ingenuity when it comes to blasting things into bloody pieces. Bedecked with steel armour? Check.
Multiple armaments, each deadlier than the last? Check that too.
Special abilities? Yep.
Looks absolutely amazing? Definitely.
The only thing that makes me hesitant about the steam tank is the fact that... well, it's a tank. It has a crew, so I don't think it really counts.
Therefore, my new favourite single entity unit is now...
The Necrofex Colossus.
I love it. It looks amazing, it's got a cannon for a hand, the way it's ramshackled together with corpses and ship hulls, it really is the coolest thing I've ever seen.
Devastating against the common rabble, decent accuracy for other single entity units, good armour and endurance (and also quite a bit of flexibility) makes this my other favourite single entity unit.