For me, it not just one that I can boil it down to.
Rimfire rifle: It would be the Remington 572 in 22LR. Far too many teenage and later shooting and hunting memories with my friends while using that rifle. Second would be my old Rem 513T, for I shot several clean 50yd smallbore comp targets and then used it for training for LR comp matches.
Centerfire rifle: My old Remington 700 BDL in 222Rem. My second purchased center fire rifle, but one of my most fired and has taken everything from crows, P-dogs, coyotes, deer, etc. At one time, it shot so well I used it in one season of hunter class BR comp. I carried that rifle more than any other CF in my collection. Second would be my old Remington 700 in 30-06 just for all the early family hunting memories attached.
Shotgun: My old Remington 870 pump. It was my first ever personally owned shotgun and a Christmas gift from my now deceased older brother when I was 15. Countless game birds and even deer were taken with it. Lots of hunting memories with friends and family.
Handgun: My Ruger Single Six 22LR/Mag. Another teenage owned firearm that went with me everywhere and shot most everything. Second would be my Dan Wesson 357mag that I purchased at the age of 20, for like the Single Six, I carried, handload and used that handgun a lot. Also, it was my first handgun to be used for serious deer hunting and then used in IHMSA revolver class.