nice one.
hmmm this is probably my absolute best one in terms of rendering:

in terms of composition maybe this one? I've got many at this level so it's hard to pick just one. some are more beautiful, whereas others are more active and engaging or interesting. I pretty much picked this one at random :P

I don't have any manga pages at my best rendering yet as my manga page style is constantly in a state of flux.
as for getting better at composition, I'm sure with practice you'll be able to expand the range of ideas you can produce, originally I just drew girls standing in t-shirts and jeans over and over again, and over time as I got more comfortable with drawing my ideas I started to branch out into more interesting ideas. If you go through my entire
art thread you can see my progress from pretty much zero.
for instantly coming up with a better idea, make sure you have a specific action your character is performing. Don't make them just pose for the camera, have them interacting with an object or another person. That will instantly make your pieces wayyyy better :P
adding a background/setting is another instant win.