It depends on the situation. Overall, I like the M60 very much. Great weapon against bigger hordes and the sound is awesome!
For short range combat and breaching doors the shotgun is fun (especially breaching doors is fun, if you have the right ammo). But I also love to deliver precision shots with the hunting rifle. In previous versions the huntiing rifle was my standard weapon in first horde nights. In A18 you don't really need it, because you find AK-47 with lots of ammo right at the start. Never liked the sniper rifle though.
My allround rifle is the AK-47 with ext. mag, silencer and reflex sight. It's quiet useful in every situation and you can clear POIs with it silently. If you equip it with 2x or 4x scopes, you have an automatic sniper rifle Headshot damage is good enough.
Oh, I really want to have a cannon!. Would love to duel myself with a buddy with a cannon from tower to tower