You want something large in calibre that can fire lots of heavy rounds in a short space of time to stop charging dangerous game. High velocity rounds like you'd be firing from an SKS are likely to pass through the animal at close range and don't transfer all their energy, so they aren't as good. That makes the 12 gauge pump or even a semi-auto, perfect.
Be aware that shotguns with slugs kick like a mule, so you'll need to practice quite a bit to get used to firing off quick, controlled shots. That is going to be reletively expensive as slugs are the most expensive type of round, but I figure if you invest for say 50 for range practice you should be right. 2 3/4 inch cartridges will be adequate and don't kick as much as 3 inchers. Also, get a gun with a good recoil pad or get one fitted. Remember to get a gun with interchangeable chokes for versatility and to only use your slugs on lite chokes like cylinder, skeet or improved cylinder - but if you do get a fixed choke gun make sure it is in one of these chokes. I'd take rifled slugs and a smooth barrel over sabot slugs and a rifled barrel as the latter are smaller and faster, so they aren't going to have the same close- range hitting power. As I understand it the semi-autos kick a little less as they use some of the gasses or recoil to re-cock the gun ... I believe Remington produced an excellent semi-auto which is discussed on a thread about shotguns on this board, but I'd be careful about getting a new Remington at the moment as they have reports of quality issues. You don't want your gun to fail in the face of a grizzly!
If you have your heart set on a rifle I'd be looking at a high calibre lever, something like a .45-70 or a .444 Marlin, though again you need to be sure that you can handle it because they kick like a horse. Also be aware that they are loud, so if you have to use them in an emergency when you don't have hearing protection get used to the idea that your ears are going to be ringing for hours afterwards, especially if they have a ported barrel, as they are loud. Really, if you are going to get one of those you need to have some practice on something smaller first so get a .22 as well. Personally I think the shotgun is the better option though, if you can only have one gun a shotgun is the most versitile with it's range limitations (and the most fun on the Sporting Clay range!).