All about play style. I tend to stick with stock, as that's always a reliable source of damage and pretty useful in most situations. I'll use the rescue ranger when I want to make aggressive plays with my sentry, as the healing and teleporting building mechanics are very helpful. The widowmaker is what I use when I'm either using a gunslinger or just want to be an aggressive engie. The frontier justice is my favorite MvM shotgun, as you pretty much just get free crits. Another use for the frontier justice is when having a mini sentry, as mini sentries are disposable so you'll probably get a crit or two. There really is no best shotgun, its all about the situation.
Oh yea, the panic attack. I would have my opinion about this, but I never use it so I don't know when to use it.