What's a good thesis statement about power of speech in Shakespeare's Julius Caesar?It has to be clear and specific.

I'm not sure how specific you need to be here, but it's
quite obvious that Mark Antony is the best example of an effective orator in
Shakespeare's Julius
Caesar. 

Immediately after Caesar's murder,
Brutus is able to persuade the angry Romans that Caesar was ambitious and that his death
was justified.  However, Mark Antony, to whom Brutus has given permission to speak, is
able to convince the Romans that Caesar was wrongly
murdered. 

If you're looking to discuss both Brutus and
Antony, you'll want to create a thesis statement that discusses ways in which they
appeal to the Romans' values (or beliefs, etc.) in Act 3.  If you want to be more
specific, you might consider something like this:  In Act 3, Mark Antony
uses verbal irony (focus on his use of the word "honorable")
and persuasion (focus on the tactis he uses to convince the
Romans that Caesar's murder was unnecesary--he shows them the stab wounds, reads them
Caesar's will, etc.) to influence the Romans' thoughts about the
conspirators' murder of Caesar. 

Hope this
helps.  Good luck with your essay!