OK, my thoughts. Apologies if I come across overly harsh:
I don't like the idea of using the old pokemon as a base for a mega, since they've usually got slightly different physiologies. Garchomp can maybe get away with it though.
The edits on this are a little sloppy though. If you're going to save yourself some effort using a base, don't be afraid in putting more effort into the edits. Garchomps fins are not the same size and shape as megachomps scythes for example, and its head is quite different.
You've got some weird photoshop effect here, which pixel art shouldn't have. It puts it way over 16 colours. It's also too cute and natural looking; Xerneas may be a deer and the embodiment of life, but it stands there very rigidly and regally, and almost looks alien up close.
BW sprites are almost all 96*96 with 15 colours + 1 transparent colour, but Kyurem-W is allowed to be larger so I think Xerneas can get away with being 20 or so colours. Just don't use that glow effect.
This one's too wobbly. The outlines just look weird, but I don't think I can get more specific.
It looks a little large but its probably fine. You have a problem with the bottom black outlines, and I think its blue is too dark for Inkay.
I don't think megalucario should be an edit of regular lucario; its about
a little bigger then it used to be. Also lucario's BW sprite is dumb.
The pose its excellent, probably the best in the thread. The colours need a little work though; some shades (the blue) need more contrast, some need more (the pink).
The shading needs work and its eyes should maybe face its opponent, but otherwise this is finished, once you save it as a png and find a better image host.
I can still instantly recognise its a pixel over though.
Pokemon in BW are standing in place, but this sprite is clearly in motion as is evident from the pose and trailing powder. Take a look at its battle animation in XY
Were these saved as jpeg? Their colours are all nasty looking and they're way above 16 colours. Never save as jpeg. Ever. The same goes for all your older pieces; I can tell they were in a proper format to start but at some point they were blurred.
Dragalga needs more work, as its darkest shade looks too close to the outline. Also Dragalga in general has a stupidly large and weedy shape so it probably will never look good as a pixel over.
Pretty good, but the outline for its cloth is jaggedy and the finer details on the hilt need work.
Also, from a flavour point of view, why is it sheathed if its in battle?
This is great. Maybe enlarge the sails and make them brighter?
It's a good sprite but its looking at the wrong angle for BW style. Compare it to pidgey or swellow and you'll see.
I don't think the sugimori art is a good base here. For one, its not looking at its opponent. Also, Vivillon's actual proportions are completely unsuited for a sprite this small, as we have a choice between body is decent sized but wings are bigger then volcorona, as it was previously, or wings are correctly sized and the body is too small to make out, as it is here. You should outright ignore the true proportions and enlarge the body quite a bit.
A more straight on pose would also make it easier to draw the various patterns; if you notice, Vivillons patterns themselves are pixel art, albeit a strange radial type of it.
The highlights on the outlines could be darker, they're too distinct from the black. The eyes are really quite misshapen.
This is fairly decent. There's nothing immediately wrong with it, but a lot of little details could be improved. The head might want some shading, the whiskers could be more distinct from the outline, the (our) leftmost earfinger could pop out a bit more. The lower body needs quite a bit of work, none of the feet or arms have very well defined shapes.
I'll see if I can find some tutorials on picking colours, since that's a common issue here. Don't be afraid to use vibrant colours and more contrasting shades.