I'm biased towards Y2K from the nostalgia, since those were the prime years of my childhood right before my teenage years kicked in.
But, I love the design of that time because of how obsessed with futurism everything was. It took the future chic look of the mid-late '60s and revamped it, taking that hype for the future- with the Space Race- bringing it back, and updating it for the Information Age.
It felt like we, as a society, had so much optimism for the world that was to come. So, if anything, I think that's what I'm mostly nostalgic for. I was so excited to grow up in that world. Damn.