What kind of yarn should I use to knit or crochet a swimsuit?

Looking to crochet (or knit) a bikini, but I need to know what kind of yarn to use and any links to sites with cute patterns would be helpful. Everyone says that swimsuits are just one of those things you shouldn’t knit because they stretch and sag, but I have seen patterns and nowdays there are so many different types of yarn, I know it’s possible. I know cotton should be avoided, and probably silk, but something with elastic, rayon, or nylon in it would probably work. Your thoughts?