I agree with Max, the thicker, black ones are the best. Lots of brands, some probably better than others. I find the one my local hardware shop is selling last for years. Yeah that yellow line tends to go hard and cracks in a fairly short space of time.
Where do you buy the black lines from? I've never had much luck finding small trimmer/mower lines that are solid, only ever got the yellow lines that end up breaking apart.