Per Wikipedia, based on capital as of 2005, Citi No.1 HSBC No.2

that is, if you trust the accuracy of Wikipedia as anyone can make input to the databank.

Bank size information
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Top ten banking groups in the world ranked by tier 1 capital
Figures in U.S. dollars, and as at end-2005[2]

Citigroup — 79 billion
HSBC — 74 billion
Bank of America — 73 billion
JP Morgan Chase — 72 billion
Mitsubishi UFJ Financial Group — 64 billion
Credit Agricole Group — 60 billion
Royal Bank of Scotland — 48 billion
Sumitomo Mitsui Financial Group — 40 billion
Mizuho Financial Group — 39 billion
Santander Central Hispano — 38 billion
[edit]
Top ten banking groups in the world ranked by assets
Figures in U.S. dollars, and as at end-2004[3]

UBS — 1,533 billion
Citigroup — 1,484 billion
Mizuho Financial Group — 1,296 billion
HSBC Holdings — 1,277 billion
Crédit Agricole — 1,243 billion
BNP Paribas — 1,234 billion
JPMorgan Chase & Co. — 1,157 billion
Deutsche Bank — 1,144 billion
Royal Bank of Scotland — 1,119 billion
Bank of America — 1,110 billion