Rock Band 2 and Beatles Rock Band were the pinnacle for me as well. Rock Band 1's instruments were poorer quality, but more importantly I didn't have a group of people to play with so it was mostly a solitary affair. By the time Rock Band 2 was out, I was living in an apartment with other people who were also into the game, and I remember many nights of playing a few songs before dinner or whatnot. One of my roommates was really into AC/DC and so of course we bought that live pack and played it a ton. At one point I bought a cheap mic stand (that broke after only a few months!) so I could play the drums and sing Spoonman at the same time. I think my roommate and I talked about doing The Ultimate Setlist (or whatever the equivalent was) in Beatles Rock Band, but we never quite carved out the time. He's in another city now, so I guess we never will.
It was also the height of Rock Band's popularity in general, so I got to participate in a charity event where people formed bands and played live Rock Band on stage at an actual venue and were judged by a panel on how awesome we were. I can probably still knock out a decent drum performance of Don't Stop Believin' by heart.
The beginning of the end was when I moved to a new apartment in a condo building and suddenly got real paranoid about making too much noise. Rock Band 3 still got some play for me, but the lack of a comprehensive career mode and the return to no-roommates living meant there was a lot less incentive to play, even as I bought the pro guitar and the keyboard and all that. Ironically, the MIDI adapter I got for the pro guitar almost never got used as a guitar adapter; instead, it allowed me to hook up a real electronic drum kit to my PS4 when Rock Band 4 came out years later.
I still worry about making too much noise, I still haven't really gotten used to the different geometry of a real kit versus the Rock Band drum kit, and it's hard to just leave the drum kit in the middle of my living room (not to mention the frankenstein laptop/USB-to-MIDI interface setup required to make it work). So after I completed Rock Band 4's career mode, I sort of shelved everything and haven't touched it since. One day, I'll get all that stuff out again, I'm sure.