Today's opening session of the United Nations General Assembly is set to be one of the most closely watched in years, not just because of the large geopolitical challenges world leaders have to tackle, but also for the eye-opening line-up.

After UN Secretary-General Ban Ki Moon opens the 70th general debate, leaders from US President Barack Obama to Iran's Mr Hassan Rouhani, Russia's Mr Vladimir Putin and China's Mr Xi Jinping will be making speeches within hours of each other.

Each speech is significant in its own right.

President Xi is making his UN assembly debut in New York and many will want to know what sort of vision he has for China's role in the global order.

There have been calls, especially from the US, for China to become a more integral player in tackling global challenges and, of late, Beijing has played a significant role in advocating for the mitigation of man-made effects of climate change and the conclusion of the landmark Iran nuclear deal.

For President Putin, the stakes are entirely different. He will likely come under some fire for Russia's annexation of Crimea last year and for recent moves that are seen as trying to set up a military outpost in Syria. Many will want to see how Mr Putin defends himself and what rhetorical attacks of his own he will launch.

President Rouhani is attending the first global summit since the nuclear deal that would see some long-held sanctions against Iran lifted in exchange for limits on its nuclear programme.

He has an opportunity in his address to try and convince a sceptical world that Iran can work well with others.

As the first of the four to speak, Mr Obama may well have the most difficult task of taking a posture of cooperation with his geopolitical rivals without coming off as being too soft on any one.

It is perhaps an indication of the fireworks in store that Cuban leader Raul Castro's first trip to the US as president is hardly making a blip on the radar.