Here's a map from the Center for American Progress showing most of the "unstable countries" that sell oil to the United States, although Venezuela's oddly left off (you'll have to click to enlarge):
That's from a new report titled "Oil Dependence is a Dangerous Habit," which estimates that the United States now spends $1 billion per day on oil imports, with most of that money going to unstable or dangerous regimes (although our two top suppliers are still Mexico and Canada). One thing to note is that, for these purposes, it doesn't really matter where we're buying from. Oil's extremely fungible—even though Iran can't legally sell crude to the United States, increased demand on our part (or shrinking supply) still benefits the regime in Tehran by driving up overall prices.
It's also worth drawing a distinction between the regimes and the countries themselves. Here's a 2007 paper by Art Durnev and Sergei Guriev discussing explanations for why oil wealth seems to be strongly associated with dismal economic performance in many poorer nations—the so-called "resource curse." Their theory? High prices cause companies in oil-related industries to reduce their transparency, especially in regions with "predatory" governments—the companies are trying to shield their assets and avoid the fate of, say, Yukos in Russia. (This isn't as big a problem in a country with healthy institutions like Norway.) And poor transparency, in turn, has all sorts of adverse effects on a country's long-run economic growth. Whether that's the reason or not, high oil prices do seem to hurt the countries you'd most expect to benefit.