Does anyone know which straightener works better to straighten hair?

It depends on your hair type, if you have thick sort of hair I'd try to find a straightener with a high heat (around 200-250c, but they can usually be expensive) if you have thin fair hair, then go for a lower heat. It's Ok to straighten your hair, just not all the time as it will damage it.

Hope I helped =)

PS. I have fair/medium hair and I use a 'Remington' straightener which has adjustable temperature, so you can change it from a low heat to high heat, they're usually pretty good =)