it's too much things to criticize: I'll try to do it one by one.
Robo Bros: Their color choices are bad. What's that supposed to be? 8-bit? 16-bit? overall it's a bad-recolor with pillow-shaded NES-megaman style Mario head.
Yoshy-Ryu 0.0: Megaman edit with a weird hairstyle. His cap is very ugly, his legs and arms should be less segmented (no big feet or arms), his punch looks odd and his projectile attacks are nothing but a simple circle-tool generated ellipsis. The colors again are bad; your hair shade is unnoticeable and it should have a shaded outline. Also, some colors you've chose are a pain on the eyes. Choose a eye-friendly color and increase your contrast. Shifting your hues and a bit of antialiasing will help, too. I suggest you studying fire so your "game blasts" won't look like paper.
Keeping it old-school: it looks more like a custom (and that's good), but the Megaman traces are still present (huge feet, walking style). I suggest you trying a custom style, if you still didn't. You need to know a bit of anatomy; some poses look really awkward.
Again, colors. The shades are more visible now, but it's still weird. Stay off from the gradient shading and use more hue-shifting: Making the dark colors a bit blue and the light colors a bit yellow will make it better. There are pillowshading in some parts: the pants look like they're glowing, and that's no good. Also, some of the things I said in the Yoshy-Ryu 0.0 section should be fixed here, too (less eyesore colors, antialiasing etc...)
G101 Alpha: Normally, splices aren't welcomed here in tSR, as it looks like a poor edit. In your case, it's very poor (sorry if I'm being a bit harsh there
) There's almost no shading on the blue and again, the outline should be shaded, too. Eyesore colors, busy face, unshaded skateboard made with MS paint tools, poor motion blur, poor resizing and pillowshading makes this sheet bad, too.
Yoshy Nitro 1: see Yoshy-Ryu 0.0. And avoid gradient shaing.
Yoshy nitro 2: see Yoshi Nitro 1. Also you should shade his belly part as a whole. If not, his belly will look 3D, and you know that Sonic's belly isn't.
Yoshy Nitro 3: Choppy, blocky, stiff, and a lot of stray pixels. I know it's his fur, but putting pixels randomly doesn't work. His gloves are totally pillowshaded. Study a bit of anatomy to make proper limbs. Also see Yoshy Nitro 1.
Scratch Yoshy Ryu: Inconstant shading. What's it? It's when you shade both his left side and his right side. Weird, isn't it? Speaking of it, there's some of these in your sprites. Also You need to work on his front poses; his face looks all messy. You don't need to mirror your stuff; we know how to do it. See anatomy and Yoshy Nitro 1.
DJ man: Anatomy, colors. Pretty much it. Try to make more smooth animations, too.
PS: and it's Danny Phantom
Yoshy Ryu NG 2: The colors are less painful in the eyes, but you need to increase contrast. It's almost without shading... Fire-looking things should have very bright colors in the middle (white, for example). His cap divisions are kinda bad but still improved from the original. Another thing that I'd like to say is: never mirror sprites. It makes your work very bad. Check your frontal poses, for example. See how his legs are. I doubt you'd like to be like that. Give it some changes and make a less boring pose.
*and change his shoes. It's too big.*
Yamato-ized sprites: Look. At. His. Hands. You made his shoes smaller but increased your hands' size...? His cap brim looks too straight, you need better colors and shading (see my other criticisms, I'm tired writing the same thing a lot of times). Also he looks anorexic in the frontal poses.
Game Man 2: The colors are lacking contrast in some areas. His outline should be shaded, or not at all. Also you could make the animations more smoother.
Jet Plasma: It could be a bit more brighter. His body looks too straight and his tail should move a bit, too.
Speech Bubbles: They look cool, but the red/orange/black ones look unshaded (or with less shading). Also that magenta one isn't good for the eyes : /
Game Man 2,5: See Game Man 2.
Yoshy Ryu 8-bit: It is NOT 8-bit. It should have 4 colors (including transparency) in a 8x8 pixel square. It should use the NES palette, too. It can be found at TSR.
Ruby: her hair is jagged, her shirt's shading is off, and some head poses look busy and weird.
Ruby 2: look Ruby.
Jock Hybrid: His legs look small compared to his body. Oh, but these colors are great
You should make palettes more like these 
I won't give CC to the bad ones. If you know they're bad, you probably know why they are.
I may have been kinda harsh, so sorry about that... I was just giving some C+C; don't get discouraged by it. I know it's a lot of things, but ya know, keep trying! It's what people say... Rome wasn't built in one day.
Robo Bros: Their color choices are bad. What's that supposed to be? 8-bit? 16-bit? overall it's a bad-recolor with pillow-shaded NES-megaman style Mario head.
Yoshy-Ryu 0.0: Megaman edit with a weird hairstyle. His cap is very ugly, his legs and arms should be less segmented (no big feet or arms), his punch looks odd and his projectile attacks are nothing but a simple circle-tool generated ellipsis. The colors again are bad; your hair shade is unnoticeable and it should have a shaded outline. Also, some colors you've chose are a pain on the eyes. Choose a eye-friendly color and increase your contrast. Shifting your hues and a bit of antialiasing will help, too. I suggest you studying fire so your "game blasts" won't look like paper.
Keeping it old-school: it looks more like a custom (and that's good), but the Megaman traces are still present (huge feet, walking style). I suggest you trying a custom style, if you still didn't. You need to know a bit of anatomy; some poses look really awkward.
Again, colors. The shades are more visible now, but it's still weird. Stay off from the gradient shading and use more hue-shifting: Making the dark colors a bit blue and the light colors a bit yellow will make it better. There are pillowshading in some parts: the pants look like they're glowing, and that's no good. Also, some of the things I said in the Yoshy-Ryu 0.0 section should be fixed here, too (less eyesore colors, antialiasing etc...)
G101 Alpha: Normally, splices aren't welcomed here in tSR, as it looks like a poor edit. In your case, it's very poor (sorry if I'm being a bit harsh there
) There's almost no shading on the blue and again, the outline should be shaded, too. Eyesore colors, busy face, unshaded skateboard made with MS paint tools, poor motion blur, poor resizing and pillowshading makes this sheet bad, too.Yoshy Nitro 1: see Yoshy-Ryu 0.0. And avoid gradient shaing.
Yoshy nitro 2: see Yoshi Nitro 1. Also you should shade his belly part as a whole. If not, his belly will look 3D, and you know that Sonic's belly isn't.
Yoshy Nitro 3: Choppy, blocky, stiff, and a lot of stray pixels. I know it's his fur, but putting pixels randomly doesn't work. His gloves are totally pillowshaded. Study a bit of anatomy to make proper limbs. Also see Yoshy Nitro 1.
Scratch Yoshy Ryu: Inconstant shading. What's it? It's when you shade both his left side and his right side. Weird, isn't it? Speaking of it, there's some of these in your sprites. Also You need to work on his front poses; his face looks all messy. You don't need to mirror your stuff; we know how to do it. See anatomy and Yoshy Nitro 1.
DJ man: Anatomy, colors. Pretty much it. Try to make more smooth animations, too.
PS: and it's Danny Phantom
Yoshy Ryu NG 2: The colors are less painful in the eyes, but you need to increase contrast. It's almost without shading... Fire-looking things should have very bright colors in the middle (white, for example). His cap divisions are kinda bad but still improved from the original. Another thing that I'd like to say is: never mirror sprites. It makes your work very bad. Check your frontal poses, for example. See how his legs are. I doubt you'd like to be like that. Give it some changes and make a less boring pose.
*and change his shoes. It's too big.*
Yamato-ized sprites: Look. At. His. Hands. You made his shoes smaller but increased your hands' size...? His cap brim looks too straight, you need better colors and shading (see my other criticisms, I'm tired writing the same thing a lot of times
). Also he looks anorexic in the frontal poses.Game Man 2: The colors are lacking contrast in some areas. His outline should be shaded, or not at all. Also you could make the animations more smoother.
Jet Plasma: It could be a bit more brighter. His body looks too straight and his tail should move a bit, too.
Speech Bubbles: They look cool, but the red/orange/black ones look unshaded (or with less shading). Also that magenta one isn't good for the eyes : /
Game Man 2,5: See Game Man 2.
Yoshy Ryu 8-bit: It is NOT 8-bit. It should have 4 colors (including transparency) in a 8x8 pixel square. It should use the NES palette, too. It can be found at TSR.
Ruby: her hair is jagged, her shirt's shading is off, and some head poses look busy and weird.
Ruby 2: look Ruby.
Jock Hybrid: His legs look small compared to his body. Oh, but these colors are great
You should make palettes more like these I won't give CC to the bad ones. If you know they're bad, you probably know why they are.
I may have been kinda harsh, so sorry about that... I was just giving some C+C; don't get discouraged by it. I know it's a lot of things, but ya know, keep trying! It's what people say... Rome wasn't built in one day.