You're just a newbie spriter, so I guess it's typical for you to make some mistakes. That's fine.
I'll skip over the Persian for now, since IMX already pointed out its flaws, and look at some of the others.
You forgot to recolor the Gardevoir's outlines. It still has green on it... did you zoom in to 6x~8x? (I'm assuming you're using Paint.) Make sure you recolor EVERY part of the sprite, otherwise it looks incomplete.
iPokemon (and similar things, like Infernos and siamese twins) irk me. They're LAZY and don't involve any creativity at all. for iPokemon, it's just "recolor the whole thing black, scratch on a poorly-done iPod, and give it a seizure-inducing Paint Default background. Please. << Spend your time doing more productive things.
The Rotom? Umm... is this a new fad or something? o_O I don't get it.
The Caterpie... as any revamping guide will tell you,
revamping is not recoloring. You can't just replace the old sprite's pallette with the new one; you have to do some work. You need to fix the outlines so they're the same as the R/S or D/P sprite, whichever you were using. Refer to
Butterfree's guide, especially the part on recoloring, to fully understand what I'm talking about.
...You realized the main problem on the Darkrai right there, but another is that your outlines are jagged, not smooth as they should be. Spriting, as you'll come to realize, takes a while. It takes work, like any piece of art should-- just because it's small-scale doesn't mean it's easy. :P Once you draw an outline, you have to refine it, make sure it's one pixel thick all the way through. Otherwise, it's jagged. You'll come to learn these things as you progress. :3
Hm... just so this post wasn't just criticism, how about a request? I'll give you an interesting mix to do for practice. Mix
Buneary and Shellos, Buneary base.