Iran is by far the biggest winner, for several reasons:
The Kurds were terribly oppressed by Saddam. Although Kurdistan became quasi-independent after the first Gulf War with American protection, Saddam was an ever present danger. Iraqi-Kurdistan was never occupied by American troops and is independent in all but name today.
The Shia have lived in Iraq for 1300 years under Sunni rule. Today the Shia control the central government and the southern oil producing regions.
al Qaeda had zero presence in Iraq before the run up to the American invasion, Saddam made sure of that (3). Today their presence is reduced from its peak but still significant. The Sunni Awakening leaders are keeping some al Qaeda fighters protected for future use against Iran and the Shia, or America if we don't leave [Engel 2008]. Also, al Qaeda's strategy is to goad America into spending ourselves into bankruptcy, and the Iraq war has added nearly a trillion dollars to our debt. Excessive debt leads to bankruptcy.
Israel suffered from suicide bombers subsidized by Saddam's Iraq. Iraq was also one of the strongest Arab countries committed to Israel's destruction. While this commitment doesn't seem to have really changed, there is no practical threat to Israel from Iraq today.
America before the Iraq invasion had an unbeatable military, a balanced budget, a strong economy and unparalleled support around the world.
The Sunnis have lost control of Iraq for the first time in 1300 years and about half of the pre-war Iraqi Sunnis are dead or have left the country. See Sunni Extermination?.
It is possible to salvage something from America's Iraq disaster. Dunkirk was a terrible defeat for the British in World War II, but most of the troops made it home even if their equipment did not. England went on to win the war, primarily by enlisting the support of America and Russia. Perhaps we can salvage something from the ashes of our disastrous Iraq adventure.
(1) Other factors reducing violence include:
US counter-insurgency field manual. One of the lessons therein: lose the moral high ground, lose the war.
(2) When the President of Iran recently visited Iraq, his visit was scheduled, he arrived in broad daylight to a red carpet, drove from the airport to the Green Zone, and traveled about with little or no security. Contrast this with how American VIPs visit Iraq. For security reasons, Americans arrive unannounced, usually at night, fly to the Green Zone in helicopters, and only travel with massive security.
(3) Saddam was our de facto ally in the war with al Qaeda, but just before the invasion Saddam allowed al Qaeda to establish a presence in Iraq. This was probably related to his strategy to make Iraq ungovernable for the U.S. [Scheuer 2008]. He also let thousands of prisoners out of jail, perhaps for the same reason.
[Engel 2008] "War journal: My Five Years in Iraq," Richard Engel, NBC News Middle East correspondent.
[Scheuer 2008] "Marching Toward Hell, America and Islam after Iraq," Michael Scheuer, former head of the CIA bin Laden unit.