Every one of these was saved as a JPEG. Never do that. Save as PNG from now on.
There's a black splotch on the left side of Aipom's head. I'm not even sure if there should be black outlining there, so you'll want to fix that. Also, you could use more black outlines on the right ear.
There aren't too many problems with Caterpie, but there's a clump of dark green outline pixels under its head. That was from the old sprite, but you need to fix it. It has the same problem near its left eye.
Mew looks okay, but you used too much black on the tail, arms, and right foot.
The Totodile looks okay, but the Rattata was just recolored and deshaded. Just because there's no shading doesn't make it a devamp; there's even shadows on the original Yellow sprite.