Despite being an Arsenal fan I used to be a season ticket holder over at Upton Park for 2-3 years as it was just around the corner.
I may be looking at this through rose-tinted goggles, but the atmosphere was fantastic, even when it only used to hold about 26,000 people once you got those fans riled up you were in for a torrid time. It completely blows away the sort of support you get at Arsenal, both at Highbury and at the Emirates. I've not been to the "London Stadium" yet, but from friends who go its just not the same.
At Upton Park the real die hard fans would sit in the Bobby Moore Lower, and that is where most of the noise would be generated. Now that sake hardcore support is segregated throughout the stadium, which also doesn't help.