Honestly, I'd say the most disastrous decisions of Obama and Biden were not really holding anyone accountable for crimes committed under the previous administration. Obama wanted to "turn the page" on Iraq and the financial meltdown while Biden never put any urgency behind the January 6th cases
What was the most disastrous decision from the most recent presidents that had effects we feel today? Clinton: Lying under oath and getting away with it (paved way for Trump) Bush: Iraq Obama: Syria and Libya Trump I: Covid and pulling out of JCPOA Biden: Running again Trump II: Iran