Aside from offending the bride, you should also avoid offending the guests. Also remember that best man speeches should not contain racial comments, religious comments, and opinions. This could bring about some tension between you and the crowd, and if this happens, they won't be laughing at you anymore. To be safe, let the groom or bride read your speech beforehand. And as a rule of thumb, when in doubt - throw it out. Do NOT say something that could be offensive. There are so many people going to be in the audience, do not take the risk. Stick to wedding quotes, wedding jokes, and making fun of the groom.