Twilight Sparkle sat around my top 6 favs since the beginning, and when Sci-twi entered the picture, I adored her so much over any other EQG character. Over time, Twilight bumped herself up my list till about this year when she squeaked by Fluttershy.
I like to envision Twilight as being without a lot of sleep, but not disturbingly so, and I don't think she'd spend a lot of time on brushing. Her OCD is more in control of her environment, not her looks, she has more important things to do with her time; so she's not as neatly trimmed as say Rarity.
I wish she wore glasses, that's a big bonus to me, one that Sci-twi carries. Both are adorkable and loveably flawed and silly in their own way, but my favorite quality has to be the intelligence and dedication to research and logic. Very difficult it seems to find similar people. Of the Mane 6, and generally any other character, I'd probably want to spend most of my time with Twilight.