You're really getting good at this
Those are really excellent, in terms of details and the thoughts put into them. I love the little touches of asymmetry here and there. The profiles are a lot less blocky and the greebles are fantastic. Really solid work
My only (very nit-picky) critiques are that:
Spoiler
This is nit-picking stuff. Really. The sprites are pretty darn good.
1. There are spots where the lighting / shadow could use a little more love. Most of it's quite good, but there are spots where it's a bit indeterminate and still has that assembled-from-parts feel- for example, the gun spots don't have any lighting, and there are a few high details that aren't shadowing lower details. Another pass on the highlighting and shadow would be helpful.
The engines in particular are feeling pretty floaty, height-wise, because the rest of the ships are pretty gray and the engines' lightness indicates they're higher than other details. I find my eye gets drawn to the engines, which (usually, anyhow) aren't supposed to be major focal points. Toning them down would probably help a bit; adding some shadows from overhanging features would probably help even more.
2. There are a few spots where the lights (in particular, the red-orange lights) could use a brighter shade to make it punchier.
And this. OK, bedtime, I'll worry about the Admiral AI tomorrow