I need to second how the "best years" perception is a sentiment influenced by one's own age, but also country of origin and individual state of mind/upbringing/tastes.
I'm a little older so late 2000s was already an era in which I was percieving new trends as a detatchment from what I knew and felt comfortable to me or my peers; on top of it a lot of stuff that's famous in my own country (or even just my region!) are irrelevant anywhere else in the world, and my school and friend groups were full of classic rock fans or diehard metalheads, so we'd often peruse media that was well before our times.
The surfacing of videos fondly talking about 2007 and related years is normal and cyclical as, obviously, the people who were younger during that era have now grown up and want to share what has mostly influenced them during childhood. Before this, we had so many essays praising the very early 2000s, the late 90s, the early 90s, the 80s... the list goes on, and it won't stop either. This is the exact same argument that has been already presented in the Frutiger Aero aesthetic thread, a lot of people are fond of it because that's what was most popular at the time of their childhood, while others prefer what was before. Soon, we'll have people being nostalgic for Metro aesthetics, and the cycle will repeat.
With all that said! I agree the mid-to-late years in a decade are especially defining as they encapsulate an establishment of all the new trends surfaced in the years prior without falling too much into the discoveries that will, in turn, define the decade right after it.
So yeah, 2007 is indeed a perfect year to sum up the 2000s as a whole! As you mentioned, early social media like YouTube (but also online galleries such as DeviantArt, SheezyArt or FurAffinity) had already come out a few years before but by 2007 they were all well established and were experiencing a "golden age" before shifting into other trends by 2010s. 2007 was also the peak of emo culture, very defining for the 2000s as a whole, though the seeds were already there in 2005 when American Idiot came out. So on and so forth!
Similarly, I could say that 1997-1998 is the perfect duo to define the cultural peak of the 90s, 1987 for the 80s...