It is definitely the Magnum!
Because it kicks ass, kills most opponents by few shots, and blasts the head of zombies in one blow.
I like the Magnum best in RE, RE2 and RE3. (to be honest I don't really like the design in RE4)
The second place goes to the Shotgun.
I like it best in allmost all the games. But especially in RE4, because holding the Shotgun makes Leon look like he seriously means business.