These are very nicely done. I like them very much, but i also do see some minor things... i hate to be the party pooper here...
On the regular lugias, In the blue around the eyes and the "fins" on it's back, there is no outlining. It would definetely give the sprite a bit more structure in those areas if you outlined the dark blue areas with at least black. I know that you had stated that you had previously used them for something else, but it still wouldn't hurt to go back and add outlining...or at least add another tone.
On dark lugia, you have some "clumpies" in the outlining of the feet. You could trim them up or at least change the tone of the inner pixels.
The only other thing that i see that bugs me are those bright white pixels over lugia's mouth in the open mouth poses. They do not appear to be teeth... so they just seem to be strange shadings. Maybe tone them down a bit?
Either way, these are all very minor details of scrutiny. These sprites are absolutely amazing, and i commend you for producing such fine pieces of art! Great job, and by all means...
Keep Spriting!