International investors are the most bullish they've been on the U.S. and Japanese markets in more than three and a half years, as both countries' economies are seen as improving, according to the latest Bloomberg Global Poll.

More than half of those contacted said the U.S. will be among the markets offering the best returns over the next year, a 15-point jump from the last poll in January and the highest rating for any country since the survey began asking that question in October 2009. Japan, which had been seen as a place to avoid in many of the previous polls, ranked second in the May 14 survey of investors, analysts, and traders who are Bloomberg subscribers, with one in three picking it as a market to favor.

"I'm upbeat on the U.S.," said Charles Doraine, president of Doraine Wealth Management Group Inc. in Corpus Christie, Texas, and a poll respondent. "Housing is coming back" and the country "can be energy independent in the not too distant future."