Favorite depends on what use you are planning to make of the gun. With that in mind here are my favorites:

Rifles
Fun: Marlin 1894 in 44 magnum
Match Shooting: M1 Garand
Social Work: AR-15

Handgun
Fun: Colt Mk IV Series 70 in .45 ACP
Match Shooting: Colt Mk IV Series 70 in .45 ACP
Social Work: Colt Mk IV Series 70 in .45 ACP
EDC: Makarov 9x18 with Makawrap grip.

Honorable Mention for just plain fun:
Ruger Bisley Vaquero in .44 Mag
Browning BDM 9mm
M1 Carbine
SKS
AK-47
All kinds of black powder weaponry.