I don't. I'm not much of a makeup or nail polish person. I like the way I look now.
When I did have nail polish I didn't use it much. Sometimes it'd just come off right after I put it on, and that drove me nuts. If it did stay on, I usually didn't have nail polish remover, so when it'd start coming off, I'd go nuts and try to take it off. Either I want a perfect coat or I don't want one at all.
Brand? Not one in particular. They're all the same for me. If I'd buy it, I'd just buy the cheapest color that caught my eye. As for colors? I'm not picky at all; the only color I don't want to wear is black. In the end, I'd just look at one and go "ooh! Pretty!" and buy that one.