Mr Miliband said the election of Bush was damning for the former PM as he discussed Blair's decision to join the US-led invasion of Iraq a decade ago, on ITV's The Agenda.
He contrasted that period with watching Blair go "through the agony" of deciding to intervene in Kosovo when Bill Clinton was in the White House.
"There's no question in my mind that if you compare that to the post-2001 period George Bush was the worst thing ever to happen to Tony Blair," he said.
"The first term of the Bush administration took America and the world on a very different course than had been set in the post Cold War period."
Miliband also called for a far stronger intervention in Syria.
"If the message to dictators around the world is that the West will never intervene - and that's the message that President Assad of Syria has got today - we're going to live in a very, very dangerous world."
The international community was "on the back foot", he said.
"The issue William Hague is raising this week - should we give more small arms - that's not really the issue because they're flooded with small arms in Syria.
"The issue is - is someone going to take on the Assad air force which is bombing his own cities?"