OPI: Probably my favorite brand of nail polish. The only con: the structure of the brush. It's pretty wide-- wider than the average brush. But it's not too bad. It's also pricey, but the quality of the polish is well worth the $8. The formula lasts, dries extremely quickly, and most of the colors are really opaque after one coat.
Sephora by OPI: This formula is a little waterier and the brush is thinner than regular OPI. Everything else is the same. However, if I were to splurge on nail polish, I would get the regular OPI.
China Glaze: My second favorite nail polish brand. The formula is fantastic. It lasts a long time while on your nails, and the brush is nice and narrow. The price also isn't too bad. If you have a Sally Beauty Supply near you, sometimes they'll have a sale; 2 for $10, or something like that. My friend E, and I usually split that deal, and each get one for $5. The only reason why it falls short of OPI is because it takes a little longer to dry.
Revlon: My favorite drugstore nail polish. It's cheap and dries fast, but may need up to 3 coats for opacity. If you're looking for a cheap and good nail polish, check this one out. The only bad thing about it is that it's kind of watery.
Forever 21: All the colors that I've tried didn't last longer than 1-2 days. If you are dying to try F21's nail polish, if anything-- I would suggest checking out any of the glitter colors. Although they're hard to take off (as all other glitter polishes are), they don't look too bad. Overall, I would not recommend this brand.
Sally Hansen: Umm... where do I even start? They have a wide selection of colors at really affordable prices, which I love. However, the thing that overpowers those pros is the fact that their formula takes decades to dry! I would not recommend this brand.
Sinful Colors: I have mixed feelings about this brand. The metallic colors, I'm not a big fan of. I bought 'Fiji', a metallic purple, last year, and have only used it once because I needed to use 5 coats to make it look like the color in the bottle. The non-metallic colors are OK, though. They last a bit longer than the F21 polishes, and are worth the $2 they are sold for. If you're looking for a fun color to wear for Halloween, or a one-night event, I would recommend this brand.
Essie: I've never used Essie, but have heard mixed opinions about it. A lot of people like it because they have a large color selection. But then again, a lot of other people hate it because supposedly, it's too "watery". I'll let you guys know if I ever try it.
In all, the best advice I can give you is to spend a few more dollars on a better brand, especially if the color is a classic one that you'd use all the time (red, nude, dark blue, any shade of pink). That way, you can have it in your collection for a while. For more trendier/fun shades that are only worn during a certain time of year or for one night, go for the cheaper brands (ex: Revlon and Sinful Colors).