I have four in the running for "my favorite".
1. First year production Remington 1100 12ga. It belonged to my Great-Grandfather, then my Grandfather, then my Uncle and then me. It is used, but still in immaculate shape, and still killing critters and clays on a regular basis.
2. Winchester 1200 12ga. The first shotgun my Father ever bought for himself. Also in great shape, also still used regularly.
3. Ithaca Model 37 Featherweight 12ga. My Father-in-law's first shotgun from when he was a kid. This thing is the ultimate pheasant killing machine!
4. Mossberg 500 12ga... The first shotgun I ever bought (well used). I have had this gun for about 17 years now, and even after a couple trips to the bottom of the lake and being dropped in just about anything you can drop a shotgun in, it still goes bang every time. It's probably the least valuable of all my shotguns, and it is definitely in the worst shape, but it has so much sentimental value I would never part with it.
I have a handfull of other scatterguns, but those four are definitely my favorites.