Your sprites are one of the best I have seen for a while now. I would like to point out a few things first.
First sprite set:
The first thing that I would like to point out on the first sprite would be the whip-like facial features. You might want to add the right whip-like feature more behind the left one mainly because it seems as if it is overlapping the left one (if you look very closely). The clump at the end should be more hidden away.
The only minor problem I found with the second sprite was that I believe you should show a minimal part of the right eye. You have space to actually make that part of the face visible. Another thing I noticed was that the right arm seems to be facing more downward, instead of actually being more dynamic and facing towards the screen (like the other arms & hands). Besides those two things, the sprite looks impressive.
Let's move on to the third one. Out of all the sprites, I believe this one is the one that needs most work. The main body needs to have more dynamic characteristics instead of being static and plain. One way you could improve that is by adding a range of colors to the main body. A few may disagree with this, but for some reason, I believe it just looks plain.
Time to head downward and talk about the fire bird. Well, since I don't have much to say about this piece, I would like to ask you if fire particles are actually present on it's head or if it is some type of spriting error? If it isn't an error, I would recommend you making it more noticeable and adding more fire.
The next few sprites have little to no problems, so I will just go through this quickly.
Make sure you make the make the light ball on it's antenna more circular.
Fire bird #2: Work on the feet a bit. They are quite difficult to make out.
The right hand on the metallic sprite needs to be curved more and you might want to make it more visible by extending out it's arm.
I can't seem to find anything wrong with the last sprite.
Again, you did an amazing job on this sprite set. There were only minimal errors, but the overall job was magnificent; however, make sure the shadow of the sprites are darker. This will give the overall piece more emphasis.
Second sprite set:
I'll go through this recent sprite set pretty quickly, because, again I only found a few minor errors.
You might want to fix the thigh on it's left leg to make it more complimentary to it's right.
Second sprite is fine, couldn't find anything wrong with it. I was a bit confused when I first saw it, because I thought it was a transparent tail, instead of two tentacle-like structures.
Now for the third sprite, I would like to mention that the front and back legs in the background need to be shaped slightly better. They seem a bit too thick to me.
Couldn't find anything wrong with the fourth sprite.
The only thing I would recommend you for the fifth sprite would be to work on its hind foot more by making it slightly more circular.
You might want to make the eye somewhat larger for the last sprite.
Definitely looking forward to seeing more sprite work from you. These were all amazing.