Like Bert says (an avid spriter here) keep it simple.
I support this statement. Lol'd, thanks FV
^ But he's totally right you know (and it's not even coincidence, he's too modest to say it but he's a pretty damn good spriter as well so you better follow his advice to the letter), your sprites are kinda over the top here and there which really puts a dent in their overall quality. So just to get the message across even clearer: KEEP. IT. SIMPLE. SILLY. (That's a K.I.S.S. for you to remember 'till you die, got it?)
Every time you add a part to a sprite, pause for a moment, count to ten, then ask yourself: "Does this piece make the sprite look better at all?". If the answer's yes, go for it. If not, ditch the damn piece.
Prime example of all this is your Bastiodon Giga Snake ...thing. The concept is cool, the Metagross arms make a great match, but PLEASE lose those stupid Charizard wings. How does a rock snake even grow flesh wings? Honestly, it's ridiculous and they make your sprite look bad. On the fiery Lairon's eyes it's basically the same thing. Yes, fiery eyes are cool, but no, it doesn't mean you can have 4 of them on the same sprite. I think it'd look way better if you made it two slightly larger flames instead of four tiny ones.
Ironically, your Corsola/Tyranitar line has the reverse problem. Too little Corsola. Hey, I'll let you in on a secret here: I made a Suicune/Corsola fusion a couple of weeks ago, and I did kinda the same thing you did. I recolored Suicune pink, made some coral crown stuff and told myself how pretty damn awesome I was. Then some friendly fellow spriter came along, told me to get off my lazy arse, and I made it look more like corsola. Here's a pic for reference (in spoilers since it's your thread and not mine):
I'd like you to do the same thing to your Tyranitar line. Blend Corsola's patterns in, make it seem like a Corsola went full steroid and became a Tyranitar.
Other than that, your Pikachu/Snorlax is great (although I think I've seen that idea before, still: nice sprite) and your aforementioned rock snake and fiery arse Lairon could be good too if you follow my advice (or do something entirely else, whatever, as long as it looks better). The Giratina/Darkrai is a mixed bag to me: I feel like you really did something right but it doesn't work out, because (imo) Darkrai's pseudo-wings/extremely pointy shoulders don't mix well with Giratina's mutha 'ucking wings.
With design stuff out of the way, here's a few technical remarks.
On your rock snake you vertically flipped Onix's body and pasted it there (middle and end part of the tail). Okay, great, but the shading is all wrong now, it looks like there's light coming out of the ground. Keep the outline, fix the shading -> your sprite will look cool and technically good + you'll get to practice your shading skills. That's two birds with one stone, yay!
Basically the same problem on your Lairon, the metal plate at its leg is poorly shaded, and I presume it is 'cause you flipped it.
Yes, this is a wall of text, but you better read it. I know where you sleep (no srsly, read it).
Yes, you'll have to work your arse off.
Do you really think I'd have written all this if you were a talentless little spriter? No way. I believe in you, you're pretty okay and we're here to make you a goddamn legend amongst mortals.
Get working
.