I'll contribute now. Even with the mass amount of physical media, shirts, stickers, and random merch, my biggest and most important possession is the tour poster from this year.
I finally had enough money (or lack of care of cost) to fly out strictly to Toronto to see the band and come back to work within 24 hours because I know my area of Canada is never going to see them for the unforeseeable future.
I met a lot of nice people there, considering I travelled alone to Toronto for the first time in my life, and a couple fans basically made me stay outside for an hour or more in the cold to meet the band. I ended up meeting 3/4 of the guys that night, and it was basically 15 years of worshipping a band encompassed in a night. I'm surprised I didn't babble like an idiot, but I had a great conversation with Hunter and Adam, who were honestly incredibly genuine and friendly.
I brought a poster tube as carry on, specifically for this purpose, and when I landed back in Saskatoon, went straight to Michaels to get this custom framed. Cost me a pretty penny, but it was worth every cent.