Interesting question--it forces the reader to consider what a real friendship is. Cassius and Brutus have a relationship that is friendly enough to include trust. As the play opens, Cassius trust Brutus enough to confide in him what he is plotting and why he wants Brutus to join in. In regards to their relationship, it seems that neither man ultimately benefits from the relationship. Both men lose everything and everyone and do not even accomplish what they set out to do.
Antony and Caesar have a relationship that would appear to be a friendship to most. However, Antony has everything to gain from his relationship with Caesar, and Caesar almost seems to play on that loyalty without truly considering Antony a friend--he treats him more like an errand boy. Antony certainly benefits from the relationship that he he established with Caesar, but again, he put himself in that position and used his speaking and leadership skills to take advantage of the turmoil created by Caesar's assassination.
There are many important friendships when it comes to Caesar. Moreover, it is false to think that Caesar could have risen to the heights of power on his own. One of the most important friendships is what scholars call the triumvirate. This basically means that there were three people who came together to help each other out in the political arena. This was especially important for Caesar, because he was the least well-known and least influential of the three men. The first man was Crassus. His claim to fame what that he was the wealthiest man in Rome. The second person was Pompey, the Great. He was the greatest general of his time. Then there was Caesar, a young and ambitious politician. Caesar was able to use the connections of these men to rise to power.