What antibotic or medicine is best for a severe case of flu?

Antibiotics vs. Anti-viral medications for Colds and Flu

Bacterial VS. Viral Infection:

Antibiotics only work against bacterial infections. They are a waste of time, money and your immune system, and can have side effects and potential allergic reactions, so they are not for treatment of a cold virus or to be used when not necessary. Unnecessary use of antibiotics is one of the main reasons we have resistant bacteria now that do not respond to treatment with some antibiotics because they have been over prescribed. Since the cold and flu are caused by viruses, antibiotics do not work for the treatment of those infections. There are some anti-viral medications available for use with the flu, but to date there are none available that provide a cure for the common cold.
The first step is being properly diagnosed. If your doctor has diagnosed you with influenza (usually based on clinical symptoms rather than laboratory testing, but sometimes using a rapid test that detects the influenza virus in your body), then there are several medicines that can be used.

For years, the drugs amantadine and rimantidine were the only agents available for treating cases of influenza. Now, however, the drugs oseltamavir (Tamiflu) and zanamavir (Relenza) are most often used. Of interest, the CDC has recently come out with a statement about their concern that one of the subtypes of influenza (Influenza A) may be resistant to Tamiflu this year. For this reason, they have recommended that cases of Influenza A be treated with Relenza. (Note: the "Swine Flu" A-H1N1/09 is not resistant to either of these anti-viral drugs and both are still being prescribed to treat the pandemic flu as of September 2009. Monitoring continues on an ongoing basis to assure effectiveness for this particular strain of flu.)

The available evidence shows that starting Tamiflu or Relenza within 24 hours of symptom onset can decrease the length and severity of symptoms.

In some cases, when a person is infected with a virus, if they have a lower immune response due to underlying conditions, they may also be given an antibiotic as a preventive measure to prophylactically treat potential secondary bacterial infections they may get due to their immune compromised conditions. Additionally, sometimes a secondary opportunistic bacterial infection will occur when a person is very ill from a virus, and in those cases, they will need treatment for the secondary bacterial condition with an antibiotic, in addition perhaps to treatment they are receiving for the viral infection. This is often the case with influenza since secondary bacterial pneumonia can be a complication of that flu.

The best defense: Vaccines

Of course, the best defense against influenza is still the flu vaccine. A vaccine for the A-H1N1/09 pandemic flu has been released in October 2009. The 2009-2010 seasonal flu vaccine is already available and the public is encouraged to get vaccinated as soon as possible for the seasonal flu and again as soon as the "Swine Flu" vaccine is made available to their risk group by the public health organizations in each community. To be protected against all the currently anticipated flu virus strains this year, vaccination with both kinds of vaccine will be required.(See related questions for additional information on flu vaccines).