I suggest you darken the outlines somewhat on the third samus to find a middle ground between it and the first. The middle samus's border has 1px lines simply stacked on top of each other; it will never look good to use that technique. Try using a style that more or less antialiases the light blue to the black over a longer distance than conventional.
Thanatos looks like he could benefit from more highlights and a less harsh style of shading. Also, consider adding just a tad of color to your greys, plain grey isn't very interesting.
The parrot is very nice, but the one to the right is lacking contrast on the green, and some darkness variation of the inner lines would make tham more suited to the size. I really like the middle one now.
The bat doesn't really have much to crit, though the animation looks technically well-executed. One suggestion, though, is to bump it up to an even 4 colors and antialias the eyes, though that might not fit well with the style. Maybe post a side-by-side comparison?
I think you did overdo the aa on the practice. You might want to use that much if you're doing a glow, as in the samus, but for a normal object it's just too much.
For stars/space, I think this piece needs to be much darker overall. Try decreasing your saturation while you're at it. Also, the hue shifting is nice, but very strong. I would make it more subtle.
You have a nice start on the sword-bear thing, but once again the hueshifting is too strong and it could use a tone between the blue and the black. Also, the eyes aren't compliant with the style you have everywhere else, being sharp and not really as smooth as other things. One last thing is that you might want to use some different palettes or subpalettes to differentiate objects.
The butterfly-thing is nice, and I like the feather texture on the wingtips, but it does seem to lack clarity of features; I'm just guessing at your intentions here. I'm not sure exactly how to correct this, but that's why it's a public forum. XD