In my opinion, you couldn't possibly go wrong with the Bushnell Banner 1.5X4.5 that several others have mentioned here. And I can only justify my opinion with fact from my own hunting.
Between me and the kids, we have 7 smoothbore slug guns, 2 in 20 gauge, 5 in 12 gauge, and all have the above mentioned Bushnell Banner mounted on them. With the wide field of view at 1.5 setting, no need for iron sites for the up close and personal shots, it's not a problem to get the scope on a moving deer at 10 feet away. And obviously the 4.5 setting is more than adequate for the outside range a slug gun is effective at.
The Bushnell Banner is EASILY the most scope you will get for the money in my opinion. Wide field of view, generous eye releif, crystal clear, they hold zero and can take the abuse dished out by a 12 gauge slug gun, we haven't had a problem with any of them. And a limited lifetime warranty if you ever do have a problem with them. All in a 70 dollar package.
My son has an Ithaca 37 12 gauge deerslayer with the Bushnell Banner mounted in a Weaver Converta mount(no longer made for the Ithaca 37, unfortunately). Last year he took a large doe at 128 lasered yards, one shot, bang flop. Not to shabby for a smoothbore slug gun with a scope most consider to 'low end' to bother with.