I like all the Sally Hansen Xtreme ones. I can put them on my toes, and they rarely chip before they start getting grown out. I don't have a favorite color though. It just depends on the season, or my mood really.
OPI. I like too many colors to have a favorite. I'm With Brad is a pretty dark red I like and Italian Wedding (if I remember correctly) is a nice bubble gum pink. There's also a great gunmetal color I love that I can't remember the name of. Sephora sells it.
China Glaze all the way! It's my favorite because it actually stays on and has a nail hardener in it. It's not too expensive and is about $6 a bottle. It comes in awesome colors, too. I've got about 70 bottles of it.