What antibiotic works best if im allergic to pennicillan and have strep throat?

Keflex: First, be sure you are really (or still) allergic to penicillin. Second, make sure you really have strep throat (some people have strep in their throat all the time!). Then your doctor can choose from several different cephalosporins. The cheapest one is the generic form of keflex. Zithromax is a distant third choice because rarely the bacteria are resistant.

Macrolides: If you have strep throat and allergic to penicillin, a macrolide anti-biotic like Azithromycin (z-pack) or Erythromycin or Clarithromycin is probably a safe and effective choice.