Contrasts need to be increased
a lot. As an example, I picked your second sprite and checked the levels around the chest area:
Let's compare that to something like Bat, shall we?
As you can see, the range of levels is much higher. If you don't know what these diagrams tell you, essentially, black is on the left, white on the right, and every other shade inbetween. The higher these peaks are, the more of that specific shade are located in the image.
This is the analytical way of telling that the shading is not strong enough. The visual one has been given by @
the mad maskman. You'll want to use a separate color for highlights and/or shadows, respectively. In this case, try something like this:
Highlights are optional. If you don't want the suit to become too bright, you can omit them (shadows need to be placed more accurately, though).
For practicing, it's great to use solid colors (->
Cel shading) because it'll give you a feeling where light hits and where it doesn't. Also, it's the general LF2-shading style because it's quite fast and reliable.
In terms of proportions, #2 & 3 are definitely more LF2-ish which would make sense going for if it's for an LF2-character. I do like the pose of #1 most, though!