I voted for FDR. No other President has ever done as much as he did to undermine the Constitution, and the principles upon which it was written. he set the stage for the federal government to grow outside the bounds that the Constitution set for it, and into the bloated, out-of-control monstrosity that it now is. I suppose it can be argued that some other Presidents since him have done more damage than he did directly, but I would say that they only did so by building upon the precedents that were set under FDR.
All the trouble that we have had since then with Islamic-based terrorism all stems from this one failure on Carter's part. Had he continued to support the Shah; or failing that, if he had taken strong military action to completely exterminate the Khomeniacs before they had the chance to spread their disease beyond the borders of Iran, none of the Islamic terrorism that we've been fighting ever since then would have happened. The 9/11 attacks would not have happened, and we would never have had any reason to have gone to war in that region.
Islamic terrorism is like a cancer, that Carter had the chance to destroy while it was still an isolated, discrete tumor. Instead, he allowed it to metastasize and spread, and now, almost four decades later, we are still suffering the increasing consequences of this failure.