TheStreet's founder and Action Alerts PLUS Portfolio Manager Jim Cramer said Berkshire Hathaway (BRK.A - Get Report) CEO Warren Buffett, who announced executive changes on Wednesday, is the greatest investor of our time.

On CNBC, Buffett said the stock market isn't so richly valued given how low interest rates are. The stock market posted its best start to a new year since 1987. 

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