Suffering from acute sinusitis, what is the best thing to take for horrible headaches?

Decongestants: The headache from acute sinusitis is due to sinus pressure, because the sinuses can't drain. While you're waiting for the antibiotics to work, using a decongestant nasal spray such as oxymetazoline 0.05% will help open up the sinuses, letting off the pressure.

OTC motrin: As well as decongestants may help. Also try sinus rinses. Afrin (oxymetazoline) is ok, but do not use that for more than 3 days in a row, you nose will get dependent/addicted to it. If the headaches persist get it checked out.