what is the best antibiotic for sinusitis?

First a very good percentage of sinusitis is viral...but if bacterial microbiology is about 38% Hflu and 38% Strep Pneumo. This is similar to AOM which is why kids with Otitis often also get sinusitis. In the real world you'd probably never get a culture unless it was really toxic or never went away. So empircally Amoxicillin is usually the first choice (like in AOM) ...then Augmentin...and if allergic Erytromycin/quinolone...etc.