This shows the addresses with the most bitcoins.
This is an outdated list of the addresses with the most bitcoins.
This doesn't prove who owns the most bitcoins, but just adds an assumption that the US government may have lots.
This also lacks the answer to the title.
A little separation seems desired. A Bitcoin wallet is defined a list of keys (usually unencrypted) where you can derive your addresses and spend from. An address is essentially the invoice they'll fund you to. Once we spend bitcoins and lock them in a way to be spendable only from the owner of the address, we announce the transaction publicly and therefore, everyone can calculate the amount of coins one address may have.
But, the amount of coins the wallet of a person holds remains unknown. You may have given me your invoice (which is your address), but that's all I know. So, you shouldn't assume the wealth of a person based on the bitcoins they own in ONE address. One may have 1,000,000 BTC in 1,000,000 addresses, but in one wallet. We couldn't understand if they're that rich.