I find this question particularly hard to answer, being right smack dab in the middle of a decade. I think I tend to romanticize my 20s because so many of my "firsts" happened during that period of my life. There have been numerous psychological studies that indicate novelty and "firsts" release dopamine and therefore make us happier. In some ways I find this true for my n of 1 case. In my 20s I:

• Travelled to more countries than ever before (Cuba, Canada, Italy, Austria, Japan, Peru, Namibia, Israel, South Africa, Zimbabwe, Brazil, Russia, the Philippines )
• Had a romantic partner who, for the first time in my life, loved me unconditionally.
• I experienced the beauty of California wilderness for the first time(Tahoe, Bodie, Burney Falls, Napa, Yosemite) and saw bears and snow leopards up close.
• I flew in a tiny 4 seater (wouldn't do again) and landed in the bush!
• I lived abroad in Southeast Asia (and hated it)
• I worked at a YC company and did the whole sleep on floors thing. In hindsight I also would not do again, but I wouldn't trade the experience for anything.
• I worked out almost every single day -- and was really in the best shape of my life.

I didn't know what I was doing and I definitely wasn't always happy (in fact I'm probably a lot happier these days), but I felt like every possibility of the universe was open to me. Like the universe was one big book, and I had infinite time to read and re-read its contents.

I do feel like time has started to take on the shape of walls these days-- cold and intimidating in many ways. I'm aware of its presence in a way I wasn't before. In this decade, I'm trying to figure out how to use that awareness in a way that pushes me to continue to live happily, freely, and without a lot of fear about the future. It's really nice to see most of the answers in this thread do not list their 20s as their favorite decade. As with everything, perspective is a beautiful thing.