My first impressions of it are that the head kind of looks a little bit too far back (so it looks a bit like he's pushing his chest forwards or such), the visible middle spike on the head looks a little bit "odd" to me (the top of it drops 1 pixel every pixel while the bottom remains at the same height, for 7 horizontal pixels in a row, which makes it look kind of triangle-like rather than rounded to me), and it sort of looks like his right leg is straight while his left leg isn't, though I know that the latter is just because of perspective and such.
If I have to be nitpicky, his nose looks kind of... weird. The shine on the top of it sort of just looks like part of the eye behind it, and the whitness at the end of it looks off compared to the darker-coloured outline edges around all of the other colours.
And the darkish-greyish spot under the left half of Sonic's eye (meaning the one towards the right of the image) also looks off to me, as it has a colour that I wouldn't really associate with neither the eye nor the muzzle, but I get that having it be whiter could make the muzzle look too slim, while having it "skin-coloured-er" would make the muzzle look too thick. I know that there's probably not much that can be done about this bit, but the long dark strip of colour there still looks a little weird to me.
But other than that, it looks very nice in my opinion (though I can't quite remember what F. Sonic's equivalent standing frame looks like, so I'm just giving my thoughts on what I think of that sprite on its own).