I wanted to do some practicing and test what kind of style variations I have. I could come up with 7 variations for the time being, but there are likely more than these. I may draw few more and post another set or update this one.
I call the first two "default" styles, because they are most natural for me to draw. In fact, I didn't really aim for these two default styles, but I like to draw them the most. These styles have just emerged after a lot of practicing. It's quite funny actually, because I don't really use them much at all. If I'm gonna draw a real manga, I believe I would go with the default style.
The next two, "usual" and "sensual" styles are what I generally want to draw. These have a lot more attention to detailing and refining. Compared to the "default" styles, these require a lot more conscious efforts to get them look good.
The "angular" and "cute" styles were sort of extremes of line handling. I found that the "angular" style fits my working methods quite well. It's also quite close to the "default" style. The "cute" style, on the other hand, doesn't seem to fit me. If I want cuteness, I prefer going with the "true default".
With the seventh variation, I wanted to try more realistic take. I hardly ever draw this kind of style, but I really like it. I like how calm the drawing turned out.
Update:Eight variant, "slender" style, is quite a different from the rest. Since I usually draw my lines quite bold, I tried drawing the lines very thin for a change. I also drew the character in shojo-like style. It turned out to be so different from the rest that I can almost consider it as an independent style rather than variation.
Number ⑨ is obviously the strongest style, so I drew "guy" style. This is purely for gags, and not very widely used independently. I however find it really fun and I hope I get to use it more in some manga. It's rather hard style because it takes a lot of understanding about the muscle anatomy, which I haven't studied enough. I just added some detailing and hope that it will fool the critical eye :D (Big Grin)

"Serious" style is a bit less extreme version of the "guy" style. I like to draw beautiful girls and then draw them with their eyes full of contempt and oppression. I mostly use it in extreme close view so that the extra detailing of the make up will be visible. This style is also a bit similar to "sensual" style, though this aims to exaggerate the "ugly" side of girls, rather than erotic.
A friend of mine requested "loli" style, so I tried doing it. I'm not sure whether it's really a variant, or just younger version of the character. The body proportions aren't only smaller, but the anatomy is also more childish. She isn't chibi, although the head might be a bit too large.
Then there are the chibis. The two versions I drew are quite different if compared, but I still lumped them together.The smaller version is frequently seen in tutorials.
I probably could have come up more than these, but I'm not sure if I could have drawn more purely unique variants. The few I tried were obvious sub-variants of these. The position of the "serious" and "loli" variants are a bit unclear to me too.
Artists in training: you don't really need to put conscious efforts in devising your style. Your style will emerge when you gain more experience and get familiar with the techniques. Just keep drawing in various styles and mixing them together. Some day you will notice that you have your very own "default" style.