Those who straighten regularly, what serum do u girls use and what works best?
For straightening hair, I find that silicone serums work best as they coat the shaft of the hair and provide some protection from the heat. That said, the product you use will depend largely on the method you use for physically straightening your hair: tongs or hairdryer.
For what it's worth: I use Hask's "Pure Shine Frizz Remedy"; it comes in an attractive glass bottle and costs about $5 at Wally World (a study of contrasts, I know).