I was kind of caught off-gaurd by this thread!
I had never realy thought about which one was my "favorite", thinking long and hard, I don't have one that stands out. The best I can do is my favorite in each category.
My favorite antique: Lefecheaux .44 pinfire revolver from my grandfather's estate.
My favorite "fun" gun/plinker: My Kel-Tec Sub9 folding carbine.
My favorite (all around) handgun: My S&W 686, because it's the most accurate in my hands.
My favorite bolt-action milsurp: My Spanish FR-8, because it feels so "handy", and it's faux CTME op-rod, and flash hider make it very "future primitive", in a Firefly/Serenity sort of way.
My favorite semi-auto rifle: My Colt "AR-15" MT6501 or whatever they were calling them…
My favorite home defense piece: My HK USP40 (It used to be my G21, but I like having an external safety on a gun I may need or "think" I need to use from a complete sleep just seconds earlier)
My favorite semi-auto: My Glock 21.
My favorite .22: MY $59 bolt-action Romanian trainer rifle. Works and shoots better than all my other .22's
My favorite shotgun: I've got only two. My Rem 870 I use for duffing in trap, and my Mossberg 590A1 for HD. Neither really gets me super-excited. If you go by use, the 870 wins, since I've busted about X10000 times more clay pigeons than home invaders.