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

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.
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.