2006 is my pick for the election that made me feel happiest, because it was a real repudiation of the authoritarian mood that had swept over the country over the preceding five years. It felt like an overwhelming victory.
Moreover, unlike 2004, when John Kerry was "reporting for duty," you had people running on full-throated critiques of the Bush administration and post-9/11 militarism. That doesn't describe every Democrat who was swept into office, obviously, but there were at least a few people winning competitive elections by saying things that people had been afraid to say in public just a few years earlier.
Just about every election since then has been a disappointment, although the repudiation of Romney-Ryanism in 2012 was a relief. By 2018, I was alienated enough from the Democratic Party not to be enthused by a mid-term victory, and, between their underwhelming gains in the Senate and the slow calls in some of their pick-ups, this did not feel like much of a victory, particularly with Trump in office. 2020 was the most nerve-wracking: By 2020, I knew that it could be close, but Trump nearly winning re-election was not something I would have predicted during his first year in office.
Something like this. 2006 was huge in that it was an open question on whether there could be an effective opposition in this country after Bush gained in 2002 and overperformed up and down the ballot in 2004 despite moving so far to the right, having an increasingly questionable war, and the economy that just wasn't very good. I thought that Bush was going to lose in 2004 simply because the economy of 2004 just didn't feel like the economy of the late 90s or even like the economy between the tech bubble and 9/11.