So I just recently made my first REAL sprite sheet. How does it look? 

Some of the poses look like recolors of the official sprites. Shading seem to fluxuate between certain actions (compare Sonic's hands when standing to Sonic's hands when crouching or running). You use the same hand pose a lot as the same hand is use for his crouching, walking, and Peel Out. Sometimes his feet doesn't have outlines and sometimes it does. The Peel Out doesn't look good (compare how it originally look to your version), Super Sonic's shading looks weird to me and the animation for a few of the poses don't look good.

 

And to me, while it does match the colors, it feels saturated. Compare how the colors look to your sprites with the Avatar you have. Study the original sheets more and see how they animate and try to match up the shading and fix a few things. It would be better looking if you could.

Thanks for the advice!
I'll try to fix it.

You might be a huge Sonic fan so I don't want this to come out of left field - but Sonic is a character that is fundamentally better designed than any of us will ever hope to admit, so I feel like Sonic spritesheets/custom sprites are a pretty huge waste of time. There's nothing really profitable to learn. These are pretty decent anyway, but

How's This?

Remember when I said study the animation? That is more than making a single quill animated. As seen in the example above, more than one quill moves for Sonic. Not just the quills as well but also his ears. Also, be sure to order the frames correctly. The animation you have looks really bad. I rearranged the frames and made it look better.



If I start at 1, the frames should be 1 - 3 - 4 - 2.

I got two this time.
Peel-out and Waiting.
I wanted the waiting to look like his foot is flopping.

well, his shoe is flopping...but like a crazy banana peel! lol

tbh, you're user icon is probably a good reference for the foot tapping. might wanna fix the floppiness

Why'd you change the shading on the head? It was fine before but now it's pillowshaded (most evident on the foot-tap).
Also, the foot-tap really only works if it's only lifted slightly off the ground like in your avatar- it's really what a foot-tap, in anything, is. What you have looks more like a floppy kick.

        Now I fixed the waiting pose and made a Win pose!

Your poses look pretty stiff. Try doing what others have been saying and STUDY the official sprites. You'll get better ideas for poses just by looking at it.

Walking!

looking better, I definitely see some improvements. i like how you added some curvature to his shoes as they move. but it looks like he has a shortened arm! (the hand behind him)

also, in the frames where his head is facing completely to the side, the blue has much better shading than in his idle stance. his head in idle is pretty pillow-shaded.

this is actually kinda bad in shading, i.e the pillow shading and overall lack of contrast. The outline does look good however for sake of readability, make his left arm extend farther to the front to make the animation more natural. Add more in between frames for the head too because it's very apparent there.

Please avoid banding and pillowshading as much as possible.

I made a new waiting based on Sonic Mania...

I think I did pretty good on this one!