Lots of very good answers here on this.
For me to give an opinion, I would have to know what type of area and distances you would expect to be defending.
For close quarters, definitely a shotgun over anything else.
Out to 100 yards, SKS, Saiga, AK47 etc. in 7.62 x39. The lever gun options are also very good and I doubt you could ever get into any trouble with a ban on those classic western carbines and they are all good out to 100 yards or farther depending on the caliber. Matching calibers for a pistol and rifle make a lot of sense.
For my non-gun enthusiast, no gun experience friends who want something simple and easy to use for home protection, I recommend a 12 guage pump, a 357 revolver, and a lever 357 carbine.
If you have to defend yourself past 100 yards, and need firepower, then I would have to say the M1A is your best choice and that is what I use in conjuction with the SKS's for my rifle battery.
If you are on a budget but want a long distance cartridge and firepower for distance, then do not overlook the Saiga .308 AK47 variant. High quality, good accuracy, and you can buy 3 of them for the price of a basic M1A.
All of this said, if you look at home defense stats worldwide, most all home defense events take place within 20' or closer. The 12 gauge shotgun is going to be the most effective gun for most home defense scenarios and should be the first gun you buy and get proficient with. Its good out to 40-50 yards with Number 1 buckshot and 100 yards with accuracy with slugs and the right shotgun.