(FORTUNE Magazine) – He has orchestrated some of the biggest mergers in history, making Citigroup the world's largest financial services firm ($1 trillion--plus in assets, market cap of $231 billion). And though Weill has decided to step aside as CEO next year (see story in First), he's kept the chairman title, made his right-hand man his replacement and will maintain a firm grip on the company until he leaves for good in 2006--make that if he leaves for good then. Under Weill, Citigroup has become the king of one-stop shopping, consistently topping its peers in lending, underwriting, and M&A advising. Weill and Citi have come under tremendous regulatory scrutiny, but he still attracts a powerful crowd. He recently threw himself a 70th-birthday party at New York City's Carnegie Hall. The event drew a mayor, a governor, a Senator, and an ex-President. --Julie Creswell